Swinburne Research Theses
The Centre is extremely proud of the achievements of its research students. CAS student theses are stored online at the Swinburne University of Technology library, please follow this link
|Student (degree)||Year||Thesis Title||Placement after Graduation|
|Russell Edwards (PhD)||2001||Pulsar Searching||University of Amsterdam|
|Willem van Straten (PhD)||2003||High-Precision Timing and Polarimetry of PSR J0437-4715||Netherlands Foundation for Research in Astronomy, Dwingeloo|
|Craig West (Masters)||2004||Development of disk-based baseband recorders and software correlators for radio astronomy||University of Massachusetts, Department of Astronomy, Amherst, USA|
|Chris Brook (PhD)||2004||Chemo-dynamical Simulations of the Milky Way||Departement de Physique, Universite Laval, Quebec|
|Stuart Gill (PhD)||2005||The Evolution of Dark Halo Substructure||Department of Astronomy, Columbia University, NY|
|Yeshe Fenner (PhD)||2005||Chemical Evolution of Galactic Systems||Harvard-Smithsonian Center for Astrophysics, Cambridge, MA|
|Michael Pierce (PhD)||2006||Spectroscopy of Extra-galactic Globular Clusters||McLennan Magasanik Associates|
|Christopher Thom (PhD)||2006||High Velocity Clouds and the Milky Way Halo||Space Telescope Science Institute, USA|
|Aidan Hotan (PhD)||2007||High-precision Observations of Relativistic Binary and Millisecond Pulsars||University of Tasmania|
|Fatma Reda (PhD)||2007||Isolated Elliptical Galaxies||National Research Institute of Astronomy and Geophysics, Cairo, Egypt|
|Tim Connors (PhD)||2008||High Resolution Simulations of Galactic Cannibalism||Australian Astronomical Observatory|
|Haydon Knight (PhD)||2008||Pulsar Applications of Baseband Recording|
|Agostino Renda (PhD)||2008||The Formation of Stellar Halos in Late-Type Galaxies|
|Trevor Mendel (PhD)||2009||Galaxy Stellar Populations and Dynamics as probes of Group Evolution||University of Victoria, Canada|
|Lee Spitler (PhD)||2009||Imaging of Extragalactic Globular Cluster Systems||Swinburne University of Technology|
|Joris Verbiest (PhD)||2009||Long-Term Timing of Millisecond Pulsars and Gravitational Wave Detection||University of West Virginia, then Max-Plank-Institut für Radioastronomie, Germany|
|Emil Lenc (PhD)||2009||Studies of Radio Galaxies and Starburst Galaxies using Wide-field, High Spatial Resolution Radio Imaging||CSIRO Astronomy and Space Science|
|Adam Deller (PhD)||2009||Precision VLBI astrometry: Instrumentation, algorithms and pulsar parallax determination||National Radio Astronomy Observatory (NRAO) Jansky Fellow / University of California Berkeley|
|Paul Kiel (PhD)||2009||Populating the Galaxy with Pulsars||Northwestern University, USA|
|Heather Alyson Ford (PhD)||2010||The H I Cloud Population in the Lower Halo of the Milky Way||University of Michigan|
|Max Spolaor (PhD)||2010||Radial Gradients in Elliptical Galaxies||Australian Astronomical Observatory|
|Sarah Burke (PhD)||2011||Supermassive Black Hole Binaries and Transient Radio Events: Studies in Pulsar Astronomy||CSIRO Astronomy and Space Science|
|Annie Hughes (PhD)||2011||Molecular Gas in the Large Magellenic Cloud||MPIA, Heidelberg|
|Caroline Foster (PhD)||2011||The Assembly and Chemical Evolution of Nearby Early-type Galaxies||ESO Fellow, Chile|
|Stefan Oslowski (PhD)||2012||The highest precision pulsar timing||Max Planck Institute for Radio Astronomy|
|Carlos Contreras (PhD)||2012||Clustering Statistics and Cosmology in the WiggleZ Survey||Carnegie/Las Campanas Observatory|
|Amr Hassan (PhD)||2012||3D Visualisation and Source Extraction for Massive Radio Astronomy Data Cubes||Swinburne University|
|Andy Green (PhD)||2012||Kinematics of Star Formation in Evolving Galaxies||Australian Astronomical Observatory|
|Emily Wisnioski (PhD)||2012||The Kinematic Properties of Clumpy Star-Forming Galaxies||MPIE Garching|
|Simon Mutch (PhD)||2012||Supercomputer models of the formation & evolution of galaxies||University of Melbourne|
|Lina Levin (PhD)||2012||A Search for Radio Pulsars: from Millisecond Pulsars to Magnetars||West Virginia University|
|Benjamin Barsdell (PhD)||2012||Advanced Architectures for Astrophysical Supercomputing||CfA, Harvard|
|Evelyn Caris (PhD)||2013||Galaxy Formation and Evolution|
|Christina Blom (PhD)||2013||The Globular Cluster System of NGC 4365||Swinburne University|
Russell Edwards, Doctor of Philosophy, 2001.This thesis reports the results of two pulsar survey projects conducted at the Parkes 64-m radio telescope in New South Wales, Australia.
The first, the Swinburne Intermediate Latitude Pulsar Survey, covered a large region of the southern Galaxy (5 degrees < |b| < 15 degrees and -100 degrees < l < 50 degrees) flanking that of the ongoing Galactic plane survey (b < 5 degrees). We used the 13-feed 20 cm "multibeam" receiver package to achieve this broad sky coverage in a short observing campaign with 14 days total integration time. The survey proved remarkably successful, detecting 170 pulsars, 69 of which were new discoveries.
Eight of the new discoveries possess small periods and period derivatives indicative of "recycling", a hypothesis supported by the fact that six of them are in circular orbits with probable white dwarf companions. Pulse time measurements have revealed that two of the white dwarfs are massive CO and ONeMg dwarfs. The mass of one of them (the companion to PSR J1157-5112) exceeds 1.14 solar masses, providing the most convincing evidence to date for the production of "ultra-massive" ONeMg white dwarfs as the end result of stellar evolution on the asymptotic giant branch (albeit with mass transfer indicated). PSR J1757-5322 also possesses a heavy (> 0.55 solar masses) white dwarf companion, in a close 11 hour orbit. The proximity of the massive companion leads to significant relativistic orbital evolution and the effects of this will be measurable by pulsar timing in the coming decades. Under general relativity, the gravitational wave power radiated from the system is sufficient to cause coalescence in < 9.5 gigayears, an event which will have dramatic and unknown consequences. Such events are possible gamma-ray burst sources, and the remnants could include isolated millisecond pulsars, close eclipsing binaries or pulsar planetary systems. The remaining four pulsar binaries show some discrepancies with the bulk of previously known low mass binary pulsars (LMBPs). PSR J1618-39 is in a 23 day orbit, filling what previously appeared to be a gap in the orbital period distribution. PSR J1745-0952 has a relatively long pulse period (19 milliseconds) and along with PSR J1618-39 (12 milliseconds) may have experienced a different evolutionary history to the majority of previously discovered LMBPs.
A ninth pulsar discovered in the survey may be recycled. The mean pulse profile of PSR J1411-7404 is exceedingly narrow (approximately 2 degrees) and lies in stark contrast to that of other pulsars of similar pulse period. In the past the only other pulsars known with anomalously narrow profiles were believed (for other reasons) to have been recycled, and this fact in combination with the low period derivative measured in timing analysis of PSR J1411-7404 leads me to suggest that it, too, may have been recycled. If this is the case, its is possible that the recycling took place in a system similar in configuration to the progenitors of the double neutron star systems but that sudden mass loss or an unfavourably oriented kick in the birth event of the second neutron star disrupted the system, leaving an isolated, mildly recycled pulsar.
The second pulsar survey program conducted for this work was a targeted search of southern globular clusters. We used a baseband recording system to provide unprecedented time resolution (typically 25 microseconds). The large number of channels and short sampling interval achievable in software filterbanks, in combination with the ability to coherently remove most of the interstellar dispersion from clusters with previously known pulsars, made use the first to achieve a relatively flat sensitivity response to pulsars of approximately 10^-3.5 - 10 seconds. This characteristic is vital if we are to constrain the true period distribution of millisecond pulsars, an important task in the evaluation of alternative equations of state for nuclear matter. We detected six millisecond pulsars and produced pulse profiles of higher resolution than were previously available. the basic sensitivity of the search was not high enough to detect any new pulsars, however the work demonstrates that the approach is feasible with the use of currently available high-preformance computing resources (such as the Swinburne workstation cluster), and is capable of delivering excellent sensitivity characteristics. It is expected that future searches of this kind, of which this is the first, will achieve the goal of sampling the true pulse period distribution within a few years.
High-Precision Timing and Polarimetry of PSR J0437-4715
Willem van Straten, Doctor of Philosophy, 2003.This thesis reports on the recent results of a continuing, high-precision pulsar timing project, currently focused on the nearby, binary millisecond pulsar, PSR J0437-4715. Pulse arrival time analysis has yielded a remarkable series of constraints on the physical parameters of this system and evidence for the distortion of space-time as predicted by the General Theory of Relativity.
Owing to the proximity of the PSR J0437-4715 system, relative changes in the positions of the Earth and pulsar result in both annual and secular evolution of the line of sight to the pulsar. Although the changes are miniscule, the effects on the projected orbital parameters are detectable in our data at a high level of significance, necessitating the implementation of an improved timing mode.
In addition to producing estimates of astrometric parameters with unparalleled precision, the study has also yielded the first three-dimensional orbital geometry of a binary pulsar. This achievement includes the first classical determination of the orbital inclination, thereby providing the unique opportunity to verify the shape of the Shapiro delay and independently confirm a general relativistic prediction.
With a current post-fit arrival time residual RMS of 130 ns over four years, the unrivaled quality of the timing data presented herein may eventually contribute to the most stringent limit on the energy density of the proposed stochastic gravitational wave background. Continuing the quest for even greater timing precision, a detailed study of the polarimetry of PSR J0437-4715 was undertaken. This effort culminated in the development of a new, phase-coherent technique for calibrating the instrumental response of the observing system.
Observations were conducted at the Parkes 64-m radio telescope in New South Wales, Australia, using baseband recorder technologies developed at York University, Toronto, and at the California Institute of Technology. Data were processed off-line at Swinburne University using a beowulf-style cluster of high-performance workstations and custom software developed by the candidate as part of this thesis.
Development of disk-based baseband recorders and software correlators for radio astronomy
Craig West, Masters of Applied Science, 2004.This thesis details work undertaken in the field of radio astronomy instrumentation. Specific components of the data collection and processing systems used by radio astronomers have been implemented using non-traditional approaches. Traditionally, the correlation of radio astronomy data has taken place on dedicated, specific hardware. This thesis deals with the implementation of equivalent correlators using software running on generic clusters of personal computers - the software approach to radio astronomy. Toward this end a system has been developed that records the raw telescope output onto computer hard drives, allowing easy access to the data on cluster supercomputers.
Part of this thesis describes the design, construction, testing and utilisation of these data recording systems. The correlator software used to process these data on supercomputers is also fully described, including extensive tests of the software and a detailed comparison between its output and the output of an existing hardware correlator. The software correlator is shown to produce output that agrees extremely well with the hardware correlator, verifying its accuracy and performance. Finally, results of on-going scientific investigations that use the software correlators described in this thesis are outlined, illustrating the exibility and usefulness of the software approach to radio astronomy.
Chemo-dynamical Simulations of the Milky Way
Chris Brook, Doctor of Philosophy, 2004.Using a state of the art galaxy formation software package, GCD+, we model the formation and evolution of galaxies which resemble our own Galaxy, the Milky Way. The simulations include gravity, gas dynamics, radiative gas cooling, star formation and stellar evolution, tracing the production of several elements and the subsequent pollution of the interstellar medium. The simulations are compared with observations in order to unravel the details of the Milky Way's formation. Several unresolved issues regarding the Galaxy's evolution are speci cally addressed. In our first study, limits are placed on the mass contribution of white dwarfs to the dark matter halo which envelopes the Milky Way. We obtain this result by comparing the abundances of carbon and nitrogen produced by a white dwarf-progenitor-dominated halo with the abundances observed in the present day halo. Our results are inconsistent with a white dwarf component in the halo > 5% (by mass), however mass fractions of 1-2% cannot be ruled out. In combination with other studies, this result suggests that the dark matter in the Milky Way is probably non-baryonic. The second component of this thesis probes the dynamical signatures of the formation of the stellar halo. By tracing the halo stars in our simulation, we identify a group of high eccentricity stars that can be traced to now-disrupted satellites that were accreted by the host galaxy. By comparing the phase space distribution of these stars in our simulations to observed high-eccentricity stars in the solar neighbourhood, we find evidence that such a group of stars - a "stellar stream" - exists locally in our own Galaxy. Our next set of simulations demonstrate the importance of strong energy feedback from supernova explosions to the regulation of star formation. Strong feedback ensures that the building blocks of galaxy formation remain gas-rich at early epochs. We demonstrate that this process is necessary to reproduce the observed low mass and low metallicity of the stellar halo of the Milky Way. Our simulated galaxy is shown to have a thick disk component similar to that observed in the Milky Way through an abrupt discontinuity in the velocity dispersion-versus-age relation for solar neighbourhood stars. This final study suggests that the thick disk forms in a chaotic merging period during the Galaxy's formation. Our thick disk formation scenario is shown to be consistent with observed properties of the thick disk of the Milky Way.
The Evolution of Dark Halo Substructure
Stuart Gill, Doctor of Philosophy, 2005.In this dissertation we analyse the dark matter substructure dynamics within a series of high-resolution cosmological galaxy clusters simulations generated with the N-body code MLAPM.
Two new halo nding algorithms were designed to aid in this analysis. The first of these was the "MLAPM-halo- finder" (MHF), built upon the adaptive grid structure of MLAPM. The second was the "MLAPM-halo-tracker" (MHT), an extension of MHF which allowed the tracking of orbital characteristics of gravitationally bound objects through any given cosmological N-body-simulation. Using these codes we followed the time evolution of hundreds of satellite galaxies within the simulated clusters.
These clusters were chosen to sample a variety of formation histories, ages, and triaxialities; despite their obvious differences, we find striking similarities within the associated substructure populations. Namely, the radial distribution of these substructure satellites follows a "universal" radial distribution irrespective of the host halo's environment and formation history. Further, this universal substructure profile is anti-biased with respect to the underlying dark matter profile. All satellite orbits follow nearly the same eccentricity distribution with a correlation between eccentricity and pericentre. The destruction rate of the substructure population is nearly independent of the mass, age, and triaxiality of the host halo. There are, however, subtle differences in the velocity anisotropy of the satellite distribution. We find that the local velocity bias at all radii is greater than unity for all halos and this increases as we move closer to the halo centre, where it varies from 1.1 to 1.4. For the global velocity bias we find a small but slightly positive bias, although when we restrict the global velocity bias calculation to satellites that have had at least one orbit, the bias is essentially removed.
Following this general analysis we focused on three specific questions regarding the evolution of substructures within dark matter halos. Observations of the Virgo and Coma clusters have shown that their galaxies align with the principal axis of the cluster. Further, a recent statistical analysis of some 300 Abell clusters confirm this alignment, linking it to the dynamical state of the cluster. Within our simulations the apocentres of the satellite orbits are preferentially found within a cone of opening angle of approximately 40 degrees around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established. Further analysis connects this distribution to the infall pattern of satellites along the filaments, rather than some "dynamical selection" during their life within the host's virial radius.
We then focused our attention on the outskirts of clusters investigating the so-called "backsplash population", i.e. satellite galaxies that once were inside the virial radius of the host but now reside beyond it. We find that this population is significant in number and needs to be appreciated when interpreting empirical galaxy morphology-environmental relationships and decoupling the degeneracy between nature and nurture. Specifically, we find that approximately half of the galaxies with current clustercentric distance in the interval 1 - 2 virial radii of the host are backsplash galaxies which once penetrated deep into the cluster potential, with 90% of these entering to within 50% of the virial radius. These galaxies have undergone significant tidal disruption, losing on average 40% of their mass. This results in a mass function for the backsplash population different to those galaxies infalling for the first time. We further show that these two populations are kinematically distinct and should be observable spectroscopically.
Finally we present a detailed study of the real and integrals-of-motion space distributions of a disrupting satellite obtained from one of our self-consistent high resolution cosmological simulations. The satellite has been re-simulated using various analytical halo potentials and we find that its debris appears as a coherent structure in integrals-of-motion space in all models ("live" and analytical potential) although the distribution is significantly smeared for the live host halo. The primary mechanism for the dispersion is the mass growth of the host. However, when quantitatively comparing the effects of "live" and time-varying host potentials we conclude that not all of the dispersion can be accounted for by the steady growth of the host's mass. We ascribe the remaining differences to additional effects in the "live" halo such as non-sphericity of the host and interactions with other satellites, which have not been modelled analytically.
Chemical Evolution of Galactic Systems
Yeshe Fenner, Doctor of Philosophy, 2005.This thesis explores the chemical signatures of galaxy formation and evolution using a software package designed specifically for this investigation. We describe the development of this multi-zone chemical evolution code, which simulates the spacetime evolution of stars, gas and a vast array of chemical elements within galactic systems. We use this tool to analyse observations of a wide range of astrophysical systems.
The chemical evolution code is first calibrated using empirical constraints from the Milky Way. These simulations help shed light on the nature of the gas accretion processes that fueled the formation of our Galaxy. We demonstrate the importance of low- and intermediate-mass stars in explaining the elemental and isotopic abundance patterns measured in Galactic stars. An intriguing question in astrophysics is whether pollution from intermediate-mass stellar winds is responsible for anomalous abundances in globular cluster stars. We test this scenario by modelling the formation and chemical evolution of a globular cluster.
Recently, the most detailed abundance pattern ever measured beyond the local universe was obtained for a high-redshift quasar absorption cloud, providing an exciting opportunity to explore early conditions of galaxy formation. We compare the chemical abundances in this distant object with predictions from a series of models, in order to gain insight into the protogalaxy's age and star formation history. We continue investigating the high-redshift universe, turning our attention to the issue of space-time variations in the ne-structure constant, as suggested by quasar absorption-line constraints. An excess abundance of heavy Mg isotopes in the absorbing clouds could partly account for the data, without needing to invoke variations in fundamental constants of nature. An enhanced early population of intermediate-mass stars could lead to such extreme Mg isotopic ratios, but we show that additional chemical consequences of this scenario conflict with observations.
Spectroscopy of Extra-galactic Globular Clusters
Michael Pierce, Doctor of Philosophy, 2006.The focus of this thesis is the study of stellar populations of extra-galactic globular clusters (GCs) by measuring spectral indices and comparing them to simple stellar population models. We present the study of GCs in the context of tracing elliptical galaxy star formation, chemical enrichment and mass assembly. In this thesis we set out to test how can be determined about a galaxy's formation history by studying the spectra of a small sample of GCs. Are the stellar population parameters of the GCs strongly linked with the formation history of the host galaxy? We present spectra and Lick index measurements for GCs associated with 3 elliptical galaxies, NGC 1052, NGC 3379 and NGC 4649. We derive ages, metallicities and alpha-element abundance ratios for these GCs using the chi-squared minimisation approach of Proctor & Sansom (2002). The metallicities we derive are quite consistent, for old GCs, with those derived by empirical calibrations such as Brodie & Huchra (1990) and Strader & Brodie (2004). For each galaxy the GCs observed span a large range in metallicity from approximately [Z/H]=--2 to solar.
We find that the majority of GCs are more than 10 Gyrs old and that we cannot distinguish any finer, age details amongst the old GC populations. However, amongst our three samples we find two age distributions contrary to our expectations. From our sample of 16 GCs associated with the 1-2 Gyr old merger remnant NGC 1052, we find no young GCs. If a significant population of GCs formed during this merger we would expect those GCs to have low mass-to-light ratios and be included in our sample of bright GCs. We find 4 young GCs in our sample of 38 around NGC 4649, an old massive cluster elliptical. There are no signs of recent star formation and therefore we do not expect any GCs to have formed within the galaxy. These results seem to indicate that the GC systems of elliptical galaxies are not strongly associated with recent field star formation. We find a correlation between the alpha-element abundance ratio and the metallicity for all three samples. Using Thomas, Maraston & Korn (2004) models, we measure much higher alpha abundance ratios for low metallicity GCs than high metallicity GCs. With current data and models we are limited in both the accuracy and the detail with which we can probe this relationship. We suggest that there are some difficulties reconciling measured GC parameters with our expectations and propose some future work which could help to resolve these and other issues.
High Velocity Clouds and the Milky Way Halo
Christopher Thom, Doctor of Philosophy, 2006.This thesis presents an exploration of stars and gas in the halo of our Galaxy. A sample of 8321 field horizontal branch (FHB) stars was selected from the Hamburg/ESO Survey. The stars make excellent tracers of the Milky Way halo, and we studied the kinematics of a subset of the HES FHB stars, comparing their velocity dispersions to those predicted by several models. Since these stars are intrinsically luminous, hot and numerous they make ideal probes of the distances to high-velocity clouds (HVCs) - clouds of neutral hydrogen gas whose distances are largely unknown and which do not fit simple models of Galaxy rotation. A catalogue of stars which align with the HVCs was developed. High resolution spectroscopy of 16 such HVC probes with the Magellan telescope has yielded a remarkably tight distance constraint to complex WB. This is one of only a handful of such distance limits so far established. Lower distance limits were set for several other clouds. Finally, we have suggested that some of the HVCs may be associated with the accretion onto the MilkyWay of the Sagittarius dwarf galaxy.
High-precision Observations of Relativistic Binary and Millisecond Pulsars
Aidan Hotan, Doctor of Philosophy, 2007.The technique of pulsar timing reveals a wealth of new information when a preci- sion of ~1µs or better is reached, but such precision is difficult to achieve. This thesis describes a series of very high precision timing observations that improve our knowledge of the targeted pulsar systems. We begin by describing a newly-developed baseband recording and coherent dedispersion system (CPSR2), along with a new object-oriented software development environment for pulsar data processing. Data obtained with this new instrument during a 3 year observing campaign at the Parkes 64 m radio telescope are analysed in a number of novel ways.
The mean profile of PSR J1022+1001 is shown to be stable on timescales of a few minutes, in contrast with previously published claims. We obtain a level of precision an order of magnitude better than any previous timing of this pulsar. In addition, we observe dramatic changes in the mean profile of the relativistic binary pulsar J1141-6545, which broadens by ~50% over the time span of our observations. This is interpreted as evidence for secular evolution of the line of sight to the emission cone, caused by General relativistic geodetic precession which tilts the spin axis of the pulsar. High precision CPSR2 observations of the extraordinary double pulsar binary system are presented and we construct calibrated, mean polarimetric profiles for PSR J0737-3039A, in two frequency bands. These profiles provide a reference against which future profile evolution may be detected, given that we expect geodetic precession to alter the observed mean profile on an even shorter time scale than for PSR J1141-6545.
The bulk of this thesis involves timing a selection of millisecond pulsars whose physical characteristics should allow the highest precision to be obtained. We mea- sure several new proper motions and parallax distances. Shapiro delay is used to constrain the inclination angles and component masses of several of the binary sys- tems in our source list. In addition, subtle periodic variations of the orbital parame- ters of two nearby binary millisecond pulsars are detected and attributed to annual orbital parallax, providing additional constraints on their three-dimensional orbital geometries. Future observations of these two sources may lead to more stringent tests of post-Keplerian gravitational theories. Finally, we use the timing residuals of one very stable source (PSR J1909-3744) as a reference against which we time PSR J1713+0747 with a root-mean-square precision of 133 ns, amongst the best timing residuals ever obtained. This result is an important step in the search for long-period gravitational waves using pulsar timing arrays.
Isolated Elliptical Galaxies
Fatma Reda, Doctor of Philosophy, 2007.This thesis presents a detailed study of a well defined sample of isolated early-type galaxies. We define a sample of 36 nearby isolated early-type galaxies using a strict isolation criteria. New wide-field optical imaging of 20 isolated galaxies confirms their early-type morphology and relative isolation. We find that the isolated galaxies reveal a colour-magnitude relation similar to cluster ellipticals, which suggests that they formed at a similar epoch to cluster galaxies, such that the bulk of their stars are very old. However, several galaxies of our sample reveal evidence for dust lanes, plumes, shells, boxy and disk isophotes. Thus at least some isolated galaxies have experienced a recent merger/accretion event which may have also induced a small burst of star formation. Using new long-slit spectra of 12 galaxies we found that, isolated galaxies follow similar scaling relations between central stellar population parameters, such as age, metallicity [Z/H] and alpha-element abundance [E/Fe], with galaxy velocity dispersion to their counterparts in high density environments. However, isolated galaxies tend to have slightly younger ages, higher metallicities and lower abundance ratios. Such properties imply an extended star formation history for galaxies in lower density environments. We measure age gradients that anticorrelate with the central galaxy age. Thus as a young starburst evolves, the age gradient flattens from positive to almost zero. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies, metallicity gradients are shallower with increasing galaxy mass. Such behaviour is expected in the remnants of multiple mergers. The metallicity gradients are also found to be correlated with the central age and metallicity, as well as to the age gradients. We generally find flat [E/Fe] gradients. We also examine the Fundamental Plane in both traditional Re, mu_e and sigma space and kappa-space. Most isolated galaxies follow the same Fundamental Plane tilt and scatter for galaxies in high density environments. However, a few galaxies notably deviate from the plane in the sense of having smaller M/L ratios. This can be understood in terms of their younger stellar populations, which are presumably induced by a gaseous merger. In conclusion, our results are compatible with an extended merger/accretion history for most isolated elliptical galaxies. However, for those galaxies which show no fine structures nor any young stellar populations, an early formation epoch followed by passive evolution is more probable.
High Resolution Simulations of Galactic Cannibalism
Tim Connors, Doctor of Philosophy, 2008.Throughout this dissertation, we will use numerical simulations to probe our understanding of Hierarchical Clustering within the local Universe. We make use of the chemodynamical simulation code GCD+, to simulate several high-resolution cosmological and galactic scale simulations. We describe a suite of analysis software used to transform the simulation data into the observer's plane, and apply it on a set of simulations that probe nearby objects being cannibalised by our own Milky Way Galaxy.
We first apply our codes to the interacting Magellanic and Milky Way system, to show that the Magellanic Stream is the result of the Magellanic Clouds being torn apart by the tidal forces between them and the Milky Way. To do this, we find the set of parameters that determine a best-fit model to the observed stream, and highlight how these parameters affect the final model. We find in particular, that the disc of the Small Magellanic Cloud is required to be larger than previous studies have shown, and we cast doubt on some recent measurements of the mass of the Large Magellanic Cloud. We also make the successful prediction of distances to some of the clouds in the stream, and several bifurcations in the stream that had not been observed prior to this work.
Applying our codes to cosmological scenarios, we find we are able to explain the existence of High Velocity Clouds as a natural byproduct of Hierarchical Clustering within the Lambda-dominated Cold Dark Matter (LCDM) paradigm. We are able to produce sufficient quantities of HI gas in such High Velocity Clouds, reflecting the known spatial and kinematical distributions of those observed, and go some of the way to reproducing other gas concentrations as well. We are able to make predictions as to their distances, placing them in the halo of the Milky Way, that are consistent also with the limited observational information thus far available.
Pulsar Applications of Baseband Recording
Haydon Knight, Doctor of Philosophy, 2008.In this thesis I report on the development and implementation of baseband recording techniques for searches for radio emission from neutron stars. Ultra-bright pulses are a sporadic type of neutron star emission that have intrinsic widths of less than a microsecond. Propagation through the interstellar medium broadens these radio pulses and consequently reduces their peak flux densities. Pre-detection dedispersion can be performed on data taken with baseband recorders to completely mitigate the dispersive aspect of this broadening. However, such ``coherent'' dedispersion is computationally expensive, and so its application has historically been limited to narrow bands and/or short observation times. This thesis describes the first wide-bandwidth searches of large data sets for ultra-bright pulses using pre-detection dedispersion techniques. The equations that determine the efficiency and sensitivity of such searches are presented, and practical data processing algorithms are discussed. The results of large-scale searches for ultra-bright pulses using the Parkes and Green Bank radio telescopes are also presented. These revealed three new emitters -- PSRs J1823-3021A, J0218+4232, and B1957+20. This increases the number of millisecond pulsars documented to emit ultra-bright pulses from two to five. In addition, millisecond pulsars that emit ultra-bright pulses are confirmed to be rare. The ultra-bright pulses from all of the five millisecond pulsar emitters are analysed in further detail. Three of these pulsars emit pulsed X-rays. The phases of the ultra-bright pulses always correlate in phase with the X-ray pulses, but have variable phase relations to ordinary radio emission. Structure in the emission of PSRs B1937+21 and J1824-2452A is seen at timescales as short as 8 and 20ns, respectively. The ultra-bright pulses of PSR J1824-2452A are found to be highly elliptically polarised. Their position angles vary widely. This may mean that local effects are dominating over the global magnetic field during the emission events. However, I also report the likely detection of an ultra-bright pulse from PSR J1823-3021A that consists of two disjoint bursts. As these must originate from widely separated regions of the pulsar magnetosphere, macroscopic excitations seem to occur during ultra-bright emission events. I also describe searches for ultra-bright pulses and periodic emission from neutron stars in globular clusters. The ultra-bright pulse searches, whilst not revealing any new emitters, place new bounds on the existence of energetic millisecond pulsars. The Fourier-domain searches of globular clusters revealed 42 previously known pulsars. One new pulsar is reported -- PSR J0024-7204Z in 47 Tucanae.
The Formation of Stellar Halos in Late-Type Galaxies
Agostino Renda, Doctor of Philosophy, 2008.Near-field observations may provide tight constraints - i.e. "boundary conditions" - on any model of structure formation in the Universe. Detailed observational data have long been available for the Milky Way (e.g., Freeman and Bland-Hawthorn 2002) and have provided tight constraints on several Galaxy formation models (e.g.: Abadi et al. 2003; Bekki and Chiba 2001). An implicit assumption still remains unanswered though: is the Milky Way a "normal" spiral? Searching for directions, it feels natural to look at our neighbour: Andromeda. An intriguing piece of the "puzzle" is provided by contrasting its stellar halo with that of our Galaxy, even more so since Mouhcine et al. (2005) have suggested that a correlation between stellar halo metallicity and galactic luminosity is in place and would leave the Milky Way halo as an outlier with respect to other spirals of comparable luminosities. Further questions hence arise: is there any stellar halo-galaxy formation symbiosis? Our first step has been to contrast the chemical evolution of the Milky Way with that of Andromeda by means of a semi-analytic model. We have then pursued a complementary approach through the analysis of several semi-cosmological late-type galaxy simulations which sample a wide variety of merging histories. We have focused on the stellar halo properties in the simulations at redshift zero and shown that - at any given galaxy luminosity - the metallicities of the stellar halos in the simulations span a range in excess of ~ 1 dex, a result which is strengthened by the robustness tests we have performed. We suggest that the underlying driver of the halo metallicity dispersion can be traced to the diversity of galactic mass assembly histories inherent within the hierarchical clustering paradigm.
Galaxy Stellar Populations and Dynamics as probes of Group Evolution
Trevor Mendel, Doctor of Philosophy, 2009.Galaxy Groups are among the most common structures in the observable universe, yet our knowledge of the specific processes acting there is limited. Given recent observations suggesting that galaxies are "pre-processed" in groups prior to entry into clusters, a more detailed study of the group environment is called for. For my Ph.D. I have obtained multi-object spectra of the NGC 5044 group using the AAOmega instrument at the Anglo-Australian Telesope. These spectra are high enough quality to not only spectroscopically confirm a significant number of galaxies as group members but also perform a detailed analysis of their stellar populations, looking at details of their star formation histories and clues to galaxy evolution in the group environment.
Imaging of Extragalactic Globular Cluster Systems
Lee Spitler, Doctor of Philosophy, 2009.
My research interest is to understand the formation and evolution of
galaxies through cosmic times. Galaxies are made up of observable
objects (e.g. stars, dust and gas), so they provide an important way
to understand the fundamental characteristics of the Universe we live
I am currently a member of the z-FourGE team, which is using the
FourStar camera on the Magellan Telescope in Chile to obtain
near-infrared images of some well-studied locations in the sky. We
are using this data to look back into time and measure very accurate
distances to galaxies when the Universe was only a few billion years
Complementary to this approach is the study of very nearby galaxies,
which have already experienced more than 10 billion years of
evolution. Our team, called SAGES, is using instruments on the Keck
and Subaru telescopes in Hawaii to obtain information about the
dynamical properties of galaxies in order to understand the dark
matter structures they reside within. Our primary observational
target are globular star clusters, which we use to probe the outer
regions of galaxies where conventional dynamical tracers are no longer
Long-Term Timing of Millisecond Pulsars and Gravitational Wave Detection
Joris Verbiest, Doctor of Philosophy, 2009.This thesis presents the results from a long-term timing campaign on 20 millisec- ond pulsars (MSPs). The stability of these pulsars is analysed in order to allow assessment of gravitational wave (GW) detection efforts through pulsar timing. In addition, we present a new method of limiting the amplitude of a stochastic back- ground of GWs and derive a strong limit from applying this method to our data. GWs are a prediction of general relativity (GR) that has thus far only been confirmed indirectly. While a direct detection could give important evidence of GW properties and provide insight into the processes that are predicted to generate these waves, a detection that contradicts GR might herald a breakthrough in gravitational theory and fundamental science. Two types of projects are currently being under-taken to make the first direct detection of GWs. One of these uses ground-based interferometers to detect the GW-induced space-time curvature, the other uses pulsar timing. This thesis is concerned with the latter: the Pulsar Timing Arrays (PTAs).
The high stability of some MSPs, along with ever increasing levels of timing preci- sion, has been predicted to enable detection of GW effects on the Earth. Specifically, it has been shown that if the timing precision on 20 MSPs can be maintained at levels of ∼100ns during five years to a decade, a correlated effect owing to GWs from predicted cosmic origins, can be detected. However, no timing at a precision of 100 ns has been maintained for more than a few years - and only on a few pulsars.
After combining archival data and employing state-of-the-art calibration meth- ods, we achieved 200 ns timing precision over 10 years on PSR J0437−4715 - which is a record at such time scales. This high stability in itself provides several interesting measurements, for example of the variation of Newton’s gravitational constant and of the pulsar mass.
We also present long-term timing results on 19 other pulsars that constitute the Parkes PTA. Our results show that most pulsars in our sample are stable and dominated by receiver noise. The potential for sub-100ns timing is demonstrated on two of our brightest sources. These timing results are used to estimate timescales for GW detection of potential PTAs worldwide and to limit the amplitude of GWs in the data. Our limit of A < 1.0×10−14 for a background with α = −2/3 is slightly more stringent than the best limit published yet.
Studies of Radio Galaxies and Starburst Galaxies using Wide-field, High Spatial Resolution Radio Imaging
Emil Lenc, Doctor of Philosophy, 2009.This thesis reports on the application of new wide-field Very Long Baseline Interferometry (VLBI) imaging techniques using real data for the first time. These techniques are used to target three specific science areas: (i) a sub-parsec-scale study of compact radio sources in nearby starburst galaxies, (ii) a study of jet interactions in active radio galaxies, and (iii) an unbiased study of the sub-arcsecond, 90 cm sky.
Six local southern starburst galaxies are surveyed on sub-parsec-scales using wide-field VLBI techniques. Compact radio sources are detected in two of the most prominent galaxies, NGC 253 and NGC 4945. Modelling of the compact source spectra reveal the majority have steep spectra, associated with supernova remnants, and are significantly free-free absorbed by a dense ionised screen. Limits on the supernova rate and star formation rate in these galaxies are estimated based on source fading, source population modelling, and on source counts and sizes. No or few compact radio sources are detected in the less prominent galaxies, presumably as a result of reduced star formation and/or star formation in sparse environments that result in weak and short-lived supernovae and remnants.
The hot spots and interaction regions of three active radio galaxies are studied at parsec-scales, for the first time, using wide-field VLBI imaging. The resulting images have provided the most detailed views of these regions to date. In two of the target sources, PKS 0518-458 (Pictor A) and PKS 0521-365, the hot spot emission is resolved into a set of compact components. The emission mechanisms in the hot spots are discussed based on their morphology and additional multi-wavelength data.
Two overlapping 28 square degree regions are surveyed in detail using wide-field VLBI techniques in the first systematic (and non-biased), deep, high resolution survey of the low frequency sky. This represents a field of view two orders of magnitude greater than anything previously attempted in a single pointing with VLBI. A total of 27 sources were detected as far as 2 degrees from the phase centre. The results of the survey suggest that new low frequency telescopes, such as LOFAR and SKA, should detect many compact radio sources and that plans to extend these arrays to baselines of several thousand kilometres are warranted.
Precision VLBI astrometry: Instrumentation, algorithms and pulsar parallax determination
Adam Deller, Doctor of Philosophy, 2009.This thesis describes the development of DiFX, the first general–purpose software cor- relator for radio interferometry, and its use with the Australian Long Baseline Array to complete the largest Very Long Baseline Interferometry (VLBI) pulsar astrometry pro- gram undertaken to date in the Southern Hemisphere. This two year astrometry program has resulted in the measurement of seven new pulsar parallaxes, which has more than tre- bled the number of measured VLBI pulsar parallaxes in the Southern Hemisphere. These measurements included a determination of the distance and transverse velocity of PSR J0437–4715 with better than 1% accuracy, enabling improved tests of General Relativity; the first significant measurement of parallax in the famous double pulsar system PSR J0737–3039A/B, and a subsequent dramatic reduction in estimates of the Galactic double neutron star system merger rate; and a factor of four revision to the estimated distance of PSR J0630–2834, which had previous appeared to possess extremely unusual x–ray emission characteristics. Additionally, the ensemble of refined distance and transverse velocity estimates have enabled a widely applicable improvement in knowledge of pulsar luminosities in several wavebands and the Galactic electron distribution at southern latitudes. Finally, the DiFX software correlator developed to enable this science has been extensively tested and verified against three existing hardware correlators and is now an integral part of the upgraded Long Baseline Array Major National Research Facility, used by astronomers throughout Australia and the world, and has been selected to facilitate a major upgrade of the world’s only full-time VLBI instrument, the Very Long Baseline Array operated by the National Radio Astronomy Observatory in the USA.
Populating the Galaxy with Pulsars
Paul Kiel, Doctor of Philosophy, 2009.Prior to this thesis no serious attempt has been made within binary system population synthesis research to model the selection effects of observational surveys. Conversely, many pulsar population models have accounted for radio survey selection effects but not detailed binary evolution. Such modelling becomes especially important when comparing theory directly to observations. In examining the factors that influence pulsar evolution, both in binary systems and as single stellar objects, we have bridged this existing gap between these two research fields. This thesis populates a model Galaxy with binary systems and evolves the population forward in time. A prediction of the Galactic pulsar population characteristics is produced, at the assumed age of the Galaxy, after we have accounted for detailed changes in stellar and binary evolution and Galactic kinematics. Synthetic observational surveys mimicking a variety of radio pulsar surveys are then performed on this population. The population synthesis synthetic survey (PS3) package is comprised of three components: stellar/binary evolution (binpop), Galactic kinematics (binkin), and survey selection effects (binsfx). The resultant pulsar populations, assuming the magnetic-dipole decay and accretion induced magnetic decay models, can compare well to many of the detected pulsar population characteristics. The comparisons between models and observations have lead to the conclusions described below. The models exclude short (∼ 5 Myr) timescales for standard pulsar exponential field decay and find that ablation of low-mass millisecond pulsar companions can redress both the lack of synthetic isolated pulsars and their excessive distances in height from the Galactic plane. Coalescing double neutron star and collapsar Galactic populations, evolved owing to standard binary evolutionary assumptions, are too centrally concentrated owing to the typical merger timescale of double neutron stars being a few million years. Dwarf galaxy models of coalescing double neutron stars and collapsars produce equally good agreement with long gamma-ray burst projected distances. Therefore our models cannot provide any distinction between which of these populations (coalescing double neutron stars or collapsars) are the progenitor of long gamma-ray bursts. The Galactic birth rate of double neutron star binaries in our model is 8.2/Myr and the merger rate is 6.8/Myr. Scintillation is found to be an important aspect in the detection of low flux density pulsars. The assumed pulsar luminosity law is found to require an inverse trend with spin period and our favoured models suggest that there are one million radio active pulsars within the Galaxy.
The H I Cloud Population in the Lower Halo of the Milky Way
Heather Alyson Ford, Doctor of Philosophy, 2010.To constrain the physical properties and distribution of the population of H I clouds in the lower halo of the Milky Way galaxy, and to provide insight into their origin and nature, the Galactic All-Sky Survey was performed and used to identify and measure properties of the H I halo clouds. Two symmetrically-located regions of the Galaxy were searched for H I halo clouds located near tangent points. The number of clouds detected within these regions are strikingly different: 255 in the first quadrant and only 81 in the fourth. The physical properties of the clouds are similar, however, suggesting that they belong to the same population and may have originated from similar environments. The cloud-to-cloud velocity dispersions are also similar, despite a factor of two difference in their vertical scale heights. This suggests that the kinematics of the clouds are driven by the same physical processes in each quadrant and that the cloud-to-cloud velocity dispersions are not responsible for the heights the clouds reach. This large, homogeneously selected sample of halo clouds has allowed their spatial distribution to be determined for the first time and has revealed that they are strongly correlated with the spiral structure of the Galaxy.
We propose a scenario where the H I halo clouds are related to areas of star formation in the form of superbubbles and gas that has been swept into the halo due to stellar winds and supernovae. This proposal was tested by performing three- dimensional hydrodynamic simulations of a superbubble within a realistic clumpy medium. The simulations revealed that it is possible for H I clouds to form in the disk-halointerface due to the evolution of a superbubble, in the form of disk gas that has been swept into the halo from the walls of chimneys, and the resulting clouds reach heights similar to those of observed clouds.
The large number of clouds detected in both regions suggest that the clouds are a major component of the Galaxy and would likely be detected throughout it. The H I halo clouds therefore play an important role in the circulation of gas between the disk and halo, and are likely prominent features in many external galaxies.
Radial Gradients in Elliptical Galaxies
Max Spolaor, Doctor of Philosophy, 2010.The aim of this Thesis is to provide a contribution to the decades-long debate regarding the formation and evolution of early-type galaxies. Our approach to this open problem is to investigate the combined kinematic, photometric, and stellar population properties at large galactocentric radii for a sample of early-type galaxies. The galactocentric radial distribution of these properties is a chemodynamical imprint of the many physical mechanisms acting in galaxies, and provides us with strong constraints on competing galaxy formation scenarios.
Initially we the derive the star formation and chemical enrichment history of two massive early-type galaxies. Our analysis is based on new high signal-to-noise long-slit spectroscopic data obtained from the ESO 3.6m telescope, and high-resolution multiband imaging data from the Hubble Space Telescope and wide-field imaging from the Subaru telescope. We derive stellar population radial profiles of age, metallicity [Z/H], and α-element abundance ratio [α/Fe] out to more than one effective radius, together with surface brightness profiles and isophotal shape parameters. The results suggest that the galaxies formed over half of their mass in a single short-lived burst of star formation at high redshift and evolved quiescently afterwards. This event likely involved an outside-in mechanism with supernova-driven galactic winds playing a fundamental role in shaping the observed steep negative radial metallicity gradients. A similar study is performed out to ∼ 1−3 effective radii for a sample of 14 low luminosity, low-mass early-type galaxies in the Fornax and Virgo clusters. We use new high-quality long-slit spectroscopic data obtained from the Gemini telescope and multiband imaging data from the Hubble Space Telescope. A gradual gas dissipation is suggested to be responsible for the old and extended stellar discs present in these galaxies. We extend our study to higher galaxy mass via a novel literature compilation of 37 early-type galaxies, which provides stellar population properties out to one effective radius. We find that metallicity gradients correlate with galactic mass, and the relationship shows a sharp change in slope at a dynamical mass of ∼ 3.5 × 1010 M⊙. We conclude that low-luminosity, low-mass galaxies likely formed in an early starforming collapse with extended, low efficiency star formation, and mass-dependent galactic outflows of metal-enriched gas. Luminous, high-mass galaxies might have formed initially by mergers of gas-rich disc galaxies and then subsequently evolved via dry merger events.
Supermassive Black Hole Binaries and Transient Radio Events: Studies in Pulsar Astronomy
Sarah Burke, Doctor of Philosophy, 2011.The field of pulsar astronomy encompasses a rich breadth of astrophysical topics. The research in this thesis contributes to two particular subjects of pulsar astronomy: gravitational wave science, and identifying celestial sources of pulsed radio emission.
We first investigated electromagnetic tracers of supermassive black hole (SMBH) binaries, which are the brightest expected source of gravitational waves for pulsar timing. We considered whether two electromagnetic SMBH tracers, velocity-resolved emission lines in active nuclei, and radio galactic nuclei with spatially-resolved, flat-spectrum cores, can reveal systems emitting gravitational waves in the pulsar timing band. We found that there are systems which may in principle be simultaneously detectable by both an electromagnetic signature and gravitational emission, however the probability of actually identifying such a system is low (they will represent ≪1% of any randomly selected galactic nucleus sample). However, this study accented that electromagnetic indicators may be used to explore binary populations down to the “stalling radii” at which binary inspiral evolution may stall indefinitely (e. g. Begelman et al. 1980) at an orbital separation exceeding those which produce gravitational radiation in the pulsar timing band. We then performed a search for binary SMBH holes, developing further the radio spectral index mapping technique and searching archival Very Long Baseline Interferometry data for 3114 radioluminous active galactic nuclei. One source was detected as a double nucleus. This result is interpreted in terms of post-merger timescales for SMBH centralisation, implications for “stalling”, and the relationship of radio activity in nuclei to mergers. Our analysis suggested that binary pair evolution of SMBHs (both of masses >108M⊙) spends less than 500Myr in progression from the merging of galactic stellar cores to within the purported stalling radius for SMBH pairs, giving no evidence for an excess of stalled binary systems at small separations. Circumstantial evidence showed that the relative state of radio emission between paired SMBHs is correlated within orbital separations of 2.5 kpc.
We then searched for transient radio events in two archival pulsar surveys, and in the new High Time Resolution Universe (HTRU) Survey. We present the methodology employed for these searches, noting the novel addition of methods for single-event recognition, automatic interference mitigation, and data inspection. A total of 27 new neutron stars have been discovered. We discuss the discoveries in terms of the relationship between “rotating radio transient” (RRAT) and pulsar populations, finding that the Galactic z-distribution of RRATs closely resembles the distribution of pulsars, and where measurable, RRAT pulse widths are similar to individual pulses from pulsars of similar period, implying a similar beaming fraction. We postulate that the RRATs may simply represent a tail of extreme-nulling pulsars that are “on” for less than a pulse period; this is supported by an investigation of the distribution of nulling and emissivity timescales for the RRAT/nulling population in general, in which we find that nulling pulsars and single-pulse search discoveries exhibit a continuous distribution across null/activity timescales and nulling fractions. We find that there is a deficit in objects with emissivity cycles longer than ∼300 seconds at intermediate and low nulling fractions which is not readily explained by selection effects, and note that the HTRU deep low-latitude survey (70-min. pointings at galactic latitudes |b| < 3.5◦ and longitudes −80◦ < l < 30◦) will be capable of exploring whether this deficit is natural or an effect of selection. The intriguing object PSR J0941–39 may represent an evolutionary link between nulling populations; discovered as an sparsely-pulsing RRAT, in follow-up observations it often appeared as a bright (10 mJy) pulsar with a low nulling fraction. It is apparent therefore that a neutron star can oscillate between nulling levels, much like mode-changing pulsars. Crucially, however, the RRAT and pulsar-mode emission sites are coincident, implying that the two emission mechanisms are linked. We estimate that the full HTRU survey will roughly quadruple the known deep-nulling pulsar population, allowing statistical studies to be made of extreme nulling populations. HTRU’s low-latitude survey will explore the neutron star population with null lengths lasting up to several hours.
We lastly reported the discovery of 16 pulses, the bulk of which exhibit a frequency sweep with a shape and magnitude resembling the “Lorimer Burst” (Lorimer et al. 2007), which three years ago was reported as a solitary radio burst that was thought to be the first discovery of a rare, impulsive event of unknown extragalactic origin. However, the new events were of clearly terrestrial origin, with properties unlike any known sources of terrestrial broad-band radio emission. The new detections cast doubt on the extragalactic interpretation of the original burst, and call for further sophistication in radio-pulse survey techniques to identify the origin of the anomalous terrestrial signals and definitively distinguish future extragalactic pulse detections from local signals. The ambiguous origin of these seemingly dispersed, swept-frequency signals suggest that radio-pulse searches using multiple detectors will be the only experiments able to provide definitive information about the origin of new swept-frequency radio burst detections.
Finally, we summarise our major findings and suggest future work which would expand on the work in this thesis.
Molecular Gas in the Large Magellenic Cloud
Annie Hughes, Doctor of Philosophy, 2011.This thesis presents new observations and analysis of the molecular gas in the Large Magellanic Cloud (LMC). The observations were conducted at the Mopra Telescope as part of the Magellanic Mopra Assessment (MAGMA) project, which has obtained high resolution (45′′) maps of the 12CO(J = 1 → 0) emission from 70% by mass of the LMC’s molecular cloud population. We show that CO emission in the LMC arises predominantly in spatially compact structures with high surface brightness, and that the total CO luminosity of the LMC is two orders of magnitude lower than would be predicted by the correlations between CO luminosity, stellar mass and 1.4 GHz radio continuum that are observed for nearby late-type galaxies.
We present a catalogue of giant molecular clouds (GMCs) in the LMC using the MAGMA CO data, and investigate whether the catalogued clouds are similar to GMCs in the Milky Way and other nearby galaxies. We find that GMCs in the LMC roughly follow the scaling relations between radius, velocity dispersion, mass and CO luminosity that have been determined for Galactic GMCs, but that LMC clouds have narrow linewidths and faint CO luminosities relative to their size. The physical properties of the observed GMCs are mostly insensitive to variations in the local interstellar conditions, but there are significant positive correlations between the atomic gas column density and the GMC velocity dispersion, and the stellar mass surface density and both the peak CO brightness and CO surface brightness of the GMCs. Our results are difficult to reconcile with models that posit molecular clouds as equilibrium structures that are regulated by either the interstellar radiation field or the ambient interstellar pressure.
Finally, we consider whether molecular gas is relevant for the correlation between the 1.4 GHz radio continuum and far-infrared (FIR) emission within the LMC. We find robust correlations between the non-thermal radio continuum and the gas and dust emission in regions covering half the LMC’s gas disk, and we identify star formation and the neutral gas surface density as the key parameters that determine the strength of these correlations. In regions where the star-formation activity is low relative to the availability of dense gas, the non-thermal radio continuum is more tightly correlated with the gas and dust emission. We demonstrate that coupling between the magnetic field strength and the gas volume density can account for the exponent of the local radio-FIR correlation that we observe in the LMC, adopting plausible assumptions for the LMC’s UV opacity, dust-to-gas ratio and cosmic ray distribution.
The Assembly and Chemical Evolution of Nearby Early-type Galaxies
Caroline Foster, Doctor of Philosophy, 2011.One of the open questions in astronomy has to do with the details of galaxy formation and evolution. My PhD project aims at constraining galaxy formation and evolution models using spectroscopic data. In order to achieve this, I first obtain 2D metallicity and kinematic maps of both the halo light and globular clusters for relatively nearby early-type galaxies out to large galactocentric radii using mainly the DEIMOS spectrograph on the Keck telescope. I also study the evolution of the overall gas phase metallicity of distant galaxies with redshift using the GAMA dataset. This compendium of information is compared to and helps constrain galaxy formation models, and provides insight into the enrichment and assembly history of galaxies throughout time.
The highest precision pulsar timing
Stefan Oslowski, Doctor of Philosophy, 2012.I am working with Matthew Bailes, Willem van Straten (both at Swinburne) and George Hobbs (ATNF). I am trying to push the limits of pulsar timing. Many pulsars can be timed to hundreds of nanoseconds. This is a very impressive result, yet still below theoretically attainable precision. One striking example is the millisecond pulsar J0437-4715, observed regularly as part of the Parkes Pulsar Timing Array. The goal of my PhD is to understand why our measurements are not as precise as we expect. This includes identifying and correcting the systematic effects in pulsar timing and improving our theoretical understanding and predictions of pulsar timing precision.
Clustering Statistics and Cosmology in the WiggleZ Survey
Carlos Contreras, Doctor of Philosophy, 2012.My thesis is about clustering statistics in the WiggleZ Dark Energy Survey. I am currently developing techniques to measure the growth rate of large-scale structure in the Universe, via the 2-point correlation function in galaxy redshift catalogues. The theoretical growth rate values at different cosmic ages are well defined for LCDM models of the Universe. Comparing these predictions with measurements is a key test of our understanding of Gravity, the force which is responsible for the existence and shape of the structures which populate the present-day Universe. Gravity also drives the peculiar velocities of galaxies, which distort the 2-point correlation function measured in redshift space. These distortions can be modelled to extract information about the tendency of matter to cluster. I am testing the robustness of my methods using simulated data, and hope to establish important constraints in the growth rate using WiggleZ data in the redshift range 0.1 < z < 1.0, contributing tools which can also be used for future galaxy surveys. I am also interested other statistical probes, such as counts-in-cells or peculiar velocity distributions, which can give important constraints on the cosmological model.
3D Visualisation and Source Extraction for Massive Radio Astronomy Data Cubes
Amr Hassan, Doctor of Philosophy, 2012.Astronomical data analysis and visualization tools need a revolutionary change to cope with the anticipated growth in the size and complexity of data sets. With large scale survey facilities capable of scanning the whole sky every few days, handling petascale datasets will be one of the major challenges for astronomy within the next decade. Consequently, the ability to handle data larger-than-memory, launch autonomous distributed analysis and visualization jobs, employ available computational accelerators, effectively deal with distributed and remote data storage facilities will no longer be a luxury.
The main target of this work is to apply scientific computing thinking and tools to design, prototype, and demonstrate a data analysis and visualization solution able to cope with the expected increase in the data size. As a member of the WALLABY survey team I focus on two main problems that astronomers will face dealing with Australian SKA pathfinder (ASKAP) extragalactic HI spectral data cubes: visualizing larger-than-memory 3D data cubes, and HI source finding in massive data cubes. I address these two problems as the main case study to demonstrate the improvements needed in astronomical data analysis and visualization tools. The enabling technology used is the graphics processing unit (GPU).
Within the first part of this work, a framework to interactively visualize larger-than-memory 3D astronomical data cubes by utilizing a heterogeneous cluster of CPUs and GPUs is presented. The framework was tested on three different CPU/GPU clusters with up to 128 GPUs, and was able to render up 560 GB spectral data cubes with an average of 8 frames/s (fps).
The second part concentrates on the problem of blind HI source finding for WALLABY. A review of different approaches used within astronomy and other fields with similar source/feature identification problems has been conducted and a new source finding technique based on envelope analysis and wavelet shrinkage was developed. The performance of the new technique was evaluated using
WALLABY test cubes. For point sources, the technique is able to provide a completeness of 81% with a reliability of 89%. For extended sources, a completeness of 63.9% with a reliability of 79.5% was obtained. For extended sources, this new approach outperforms the current ASKAP planned source finder, Duchamp. The new technique introduces the concept of using integrated flux as a thresholding criteria in addition to the peak flux value. Also, it provide an automated noise reduction mechanism using wavelet shrinkage and with limited user intervention in the de-noising process.
Through the work presented in this thesis, the WALLABY survey team is better-placed to overcome the technical challenges imposed by ASKAP, allowing them to focus on scientific outcomes.
Kinematics of Star Formation in Evolving Galaxies
Andy Green, Doctor of Philosophy, 2012.This work explores how the kinematics of star forming galaxies in the modern epoch compare with earlier galaxies. It focuses on what physical processes are responsible for the differences in galaxies between epochs. It also validates new observational techniques across a large range in kinematic properties. Previous works have found early galaxies to be highly turbulent, in marked contrast to modern galaxies. Those works argue the accumulation of stellar mass and the changing gas accretion rates drive the evolution of galaxies between early and modern states. Theoreticians have postulated several mechanisms of galaxy assembly, which can explain the observed evolution. Debate centres around exactly which physical processes give rise to the kinematic states of observed galaxies, whether the processes differ with epoch, and how observations bias the observations.
This thesis explores a broader range in kinematic states in modern galaxies than previously considered in a single sample. A simple selection from a large sample of galaxies makes this range possible. Integral field spectroscopy provides observations commensurate with previous work. A handful of galaxies in this sample show kinematics very similar to galaxies observed at early epochs, while the remainder are more representative of modern galaxies. This work also finds star formation rate and gas turbulence are closely linked in galaxies at all epochs, but these two phenomenon are not always spatially coincident within galaxies. It identifies highly turbulent, clumpy star forming disk galaxies in the modern Universe---objects previously thought non-existent. This work also validates, in a controlled environment, the new observational techniques commonly used on early galaxies. The continued presence of highly turbulent disk galaxies in the modern epoch provides new constraints on galaxy evolution models. The previously unknown correlation between star formation and turbulence in galaxies indicates the important physical link between these two processes. These results provide new constraints for future models of galaxy evolution.
The Kinematic Properties of Clumpy Star-Forming Galaxies
Emily Wisnioski, Doctor of Philosophy, 2012.This thesis presents a multiwavelength survey of Halpha luminous star-forming galaxies at z~1.3 selected from the WiggleZ Dark Energy Survey. It features data from near-infrared integral field spectroscopy (IFS) aided by laser guide star adaptive optics along with far-infrared space-based imaging. The objects studied are representative of the most Halpha luminous galaxies at this redshift, providing useful testbeds to study the triggers of rapid star formation, such as merger events or internal processes. Morphologies of star-forming galaxies at z>1 are typically `clumpy' and irregular, a signature often associated with merging systems. However, the abundance of irregular morphologies observed can not be explained theoretically by mergers alone. Recent observational evidence and theoretical models suggest that some of the clumpy galaxies are governed by disk-like rotation.
The combination of kinematics, morphologies, and infrared luminosities for the sample presented here indicates that the different stages of unstable clumpy disk formation at z>1 can be traced by Halpha luminous galaxies. Within this sample, we have uncovered both clumpy and smooth disk galaxies as well as compact galaxies reminiscent of pseudo-bulges. Within the clumpy galaxies the properties of individual star-forming regions, resolved with adaptive optics, were measured from the IFS data. These regions, or clumps, are found to follow tight scaling relations with local HII regions and other high-redshift clumps for Halpha size, velocity dispersion, luminosity and mass over the range z=0-2. The results of this thesis suggest that star-forming galaxies at z~1.3 are fed gas by cold dense streams from the intergalactic medium that drives-up disk velocity dispersion resulting in the large clumps measured within these systems.
This study presents one of the largest high-redshift IFS samples observed with adaptive optics. It explores the nature of `main sequence' star-forming galaxies that are observed at various stages in their formation. However, even with the combination of integral field spectroscopy and multi wavelength data spanning from the ultraviolet to the far-infrared, the nature of some galaxies in the sample remains unresolved. Future work, connecting the molecular gas with current observations of ionised gas, could more directly probe the triggers of rapid star-formation by revealing the available gas reservoirs for these galaxies. More extensive multiwavelength datasets and advances in instrumentation, such as integral field spectrographs and adaptive optics, will greatly benefit future kinematic studies of star-forming galaxies.
Supercomputer models of the formation & evolution of galaxies
Simon Mutch, Doctor of Philosophy, 2012.In this thesis, we explore the use of semi-analytic galaxy formation models and related techniques as a means to investigate the physics of galaxy formation and evolution. We begin by investigating the ability of a highly cited semi-analytic model to reproduce the evolution of the observed galaxy population over the last 7 billion years of cosmic time. This is achieved by carrying out a detailed statistical calibration of the model's free parameters in order to simultaneously reproduce the observed galactic stellar mass function at z=0 and z0.8, as well as the z=0 black-hole bulge relation. In order to be successful, we are required to push the parameters of the model associated with supernova feedback to implausibly high values, suggesting that the current implementation of this physical prescription may be inadequate. Additionally, we suggest that some extra mechanism is required to preferentially increase the efficiency of star formation in the most massive galaxies at high redshift. In order to further explore the utility of semi-analytic models, we then present their novel use as a tool to investigate the current evolutionary status of the Milky Way and M31. The Milky Way is the most closely studied galaxy in the Universe. However, to understand the Milky Way's place in the broader landscape of galaxy evolution, we require a baseline population of galaxies against which to compare. Using a sample of analogue galaxies drawn from both observational data and semi-analytic models we find that both the Milky Way and M31 may be 'green valley' galaxies undergoing an important evolutionary transition. Furthermore, using the histories of our model analogue sample, we investigate the possible physical mechanisms which could be driving this evolutionary change. Finally, we introduce a new, self-consistent model for connecting the growth of galaxies to the formation history of their host dark matter halos. This model dispenses with attempts to implement all of the dominant baryonic processes associated with galaxy evolution and replaces them with two simple, phenomenologically motivated equations that depend only on a single host halo property. We demonstrate the ability of our new formation history model to reproduce the observed red and blue stellar mass functions at z=0. Then, by adding a simple redshift dependence to the parameterisations, we show that it can also successfully match the observed global mass functions out to z=4. To conclude, we highlight both the advantages of this model over the relevant alternatives and its general utility.
A Search for Radio Pulsars: from Millisecond Pulsars to Magnetars
Lina Levin, Doctor of Philosophy, 2012.The southern High Time Resolution Universe (HTRU) survey for pulsars and fast transients is currently underway at the Parkes Radio Telescope in Australia. In this thesis we describe a search for pulsars in the intermediate latitude part (medlat) of this survey, which is now complete. With higher time and frequency resolution than previous surveys carried out in this region of the sky, the medlat survey was more sensitive to millisecond pulsars (MSPs) at higher distances from the Sun. We discuss the survey strategy and set up, the instrumentation used to collect data as well as the processing pipeline and search techniques used to discover pulsars in the data set. In total this survey has returned 104 newly discovered pulsars, including 26 MSPs. Some of these pulsars are particularly interesting, including an MSP with a Jupiter-mass companion, an eclipsing binary pulsar and an MSP suitable for the Pulsar Timing Array projects.
A major find from the HTRU medlat survey was the radio magnetar PSR J1622-4950, which is described in detail in this thesis. Magnetars are slowly rotating neutron stars with extremely large surface magnetic field strengths, that occasionally undergo large outbursts of radiation. PSR J1622-4950 is one of 23 known magnetars and one of only 3 to have confirmed radio pulsations. The radio magnetars differ from ordinary rotation-powered pulsars e.g. by their highly variable flux densities, changing pulse profiles and flat radio spectrum. Evidence of a decrease by a factor of ~2 over the last 700 days in the peak flux density of PSR J1622-4950 is observed, indicating a transient nature. A fit of the rotating vector model to the position angle of the linear polarisation of the emission from the magnetar suggests a nearly aligned geometry of the pulsar.
Finally, we have used the large number of MSPs discovered in the HTRU medlat survey to carry out a population study of MSPs in the Galaxy. Pulsar population studies make use of the observed properties of the known pulsar sample to derive the numbers and properties of the underlying Galactic population of pulsars. We have performed a large-scale flux-limited pulsar population synthesis study. At the derived best estimate of the scale height, z = 500 pc, the simulation implies an underlying population of ~150,000 MSPs with luminosities above L_1400 = 0.07 mJy kpc^2. The results from the simulation predict that the two remaining parts of the southern HTRU survey will discover ~85 previously unknown MSPs.
Advanced Architectures for Astrophysical Supercomputing
Benjamin Barsdell, Doctor of Philosophy, 2012.Scientists, especially astronomers, are always searching for more computing power with which to run their latest simulations or reduce their latest data sets. While the speeds of traditional computers (i.e., CPUs) continue to increase, some researchers have begun to exploit alternative computing architectures. One such architecture, which we can thank the booming video-gaming industry for, is the Graphics Processing Unit (GPU). Relative to current CPU hardware, GPUs provide theoretical speed-ups of over 100x, and for certain problems such numbers have already been measured. But exactly which problems are likely to see these kind of performance boosts? The architecture of a GPU is quite different to that of a traditional CPU, and translating a problem for the GPU can be a challenging task. My work with David Barnes and Chris Fluke takes a step back from ad-hoc GPU implementations of common problems in astronomy and analyses the underlying algorithms, with the goal of being able to categorise problems for which GPU-efficient algorithms exist. Once we have an understanding of which problems are suitable for GPU implementation, the task of making efficient use of GPUs can be generalised. Further, the work will apply not only to GPUs, but to all future computer architectures. It is my hope that many areas of astronomy and astrophysics will benefit from both this analysis and the increased computing power offered by advanced computing architectures.
Galaxy Formation and Evolution
Evelyn Caris, Doctor of Philosophy, 2013.Our understanding of the morphological evolution of galaxies has advanced significantly with deep imaging surveys. Recently, a puzzling population of distant galaxies was discovered. These so-called "red nuggets" are characterised by their compact size and old ages. Interestingly they do not appear to exist in the nearby Universe posing a unique challenge for the standard models of galaxy evolution. I am currently studying these galaxies with Prof. Karl Glazebrook to improve their statistics and knowledge, and perhaps solve their mystery!
The Globular Cluster System of NGC 4365
Christina Blom, Doctor of Philosophy, 2013.This thesis presents a study of the globular cluster (GC) system of NGC 4365 and the GCs associated with the $W'$ group of galaxies, of which NGC 4365 is the dominant galaxy. We use the analysis of these GC systems to investigate the evolutionary history of NGC 4365 as well as the ongoing interactions within the $W'$ group.
To analyse the photometric properties of NGC 4365's GC system we combined three filter imaging from the SuprimeCam instrument on the 8m Subaru telescope with eight, two filter, pointings from the Advanced Camera for Surveys on the Hubble Space Telescope. To analyse the kinematic properties of NGC 4365's GC system we obtained spectra for over 250 GCs around NGC 4365 from the DEep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. The photometric properties of the group GCs were analysed with square degree, three filter imaging from the MegaCam instrument on the Canada-France-Hawaii Telescope.
We confirmed that NGC 4365 hosts three GC subpopulations, the usual blue and red GC subpopulations plus an additional subpopulation at intermediate colour: the green subpopulation. Photometric analysis showed that the three subpopulations have distinct radial and azimuthal distributions, different median sizes and mass distributions drawn from different populations. Using recession velocities calculated from GC spectra we also determined that each GC subpopulation rotates about the galaxy in a different direction. Analysis of the spatial distribution of GCs around NGC 4365 in the wider $W'$ group environment uncovered a stream overdensity of GCs between NGC 4365 and a nearby small lenticular galaxy NGC 4342, extending South West beyond NGC 4342. This GC stream is spatially coincident with a stellar stream recently presented in the literature. We found that the recession velocities of the stream GC are consistent with the recession velocity measured from NGC 4342's starlight.
We conclude that NGC 4365 formed two GC subpopulations during two separate in situ, dissipative star formation episodes (the green GCs forming before the red GCs) and accreted the blue GCs from smaller galaxies throughout its evolutionary history. We also conclude that NGC 4365 is currently accreting blue GCs and stars from the small lenticular galaxy NGC 4342. As predicted by the hierarchical merging models of $Lambda$CDM, we have observed evidence of ongoing growth of the giant elliptical galaxy NGC 4365 in the nearby Universe.