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Colloquia Series

For more information on colloquia at the Centre for Astrophysics and Supercomputing please contact Dr. Nikole Nielsen ()

Swinburne Virtual Reality Theatre
Applied Science Building, Room 406
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2001 Colloquia


Nov. 29 @ 16:00
James Rose (North Carolina)
Title: Multiple Merging Events in the Double Galaxy Cluster A3128/A3125
In the most currently popular model for the evolution of structure in the universe, structure on all scales builds up through a hierarchical sequence of mergers from a primordial density perturbation spectrum, amplified by gravity after the epoch of recombination. Thi scenario has been successful at reproducing a number of properties regarding the statistical distribution of galaxies, but a definitive test remains elusive. In this seminar, data will be presented on the variety of substructures present in the A3128/A3125 double galaxy cluster, which is itself embedded in the massive Horologium-Reticulum Supercluster. A combination of multi-fiber spectroscopic data from the AAT/2dF, X-ray imaging with the Chandra ACIS-I, and radio imaging with MOST and the ATCA will be presented. Evidence for the existence of substructure on a variety of scales will be discussed, from transient disturbances in the hot intracluster medium to a 4000 km/s underpopulated redshift zone on both sides of the cluster mean redshift. Evidence will also be discussed for a very high (hypersonic) infall velocity of merging substructures, due to the gravitational influence of the Supercluster.
Nov. 27 @ 16:00
D.J. Pisano (ATNF)
Title: Assembling Isolated Galaxies
I will present the results of an HI and optical survey of a sample of 41 isolated galaxies in a search for the gaseous remnants of the galaxy assembly process. Such remnants are predicted to exist by models of hierarchical galaxy formation and may still be falling onto these galaxies today. There are a total of 13 low-mass, gas-rich companions around 10 of the sample galaxies, which could be associated with galaxy assembly. A few more galaxies have asymmetric HI distributions, which could indicate a recent accretion event. Using this data, we have derived an HI mass function, and I will discuss its implications for the nature of high-velocity clouds. We also compared our data with predictions of cold and warm dark matter models of galaxy formation, and I will address the implications of our data on these models. Finally, I will partially answer the question: has galaxy formation concluded?
Nov. 26 @ 16:00
Ariane Lancon (Strasbourg)
Title: Interpreting Starburst Spectra
I will present an interpretation of the UV+optical+near-IR spectra of starburst galaxies, based on the example of NGC7714. I will discuss the relative weights of young, intermediate age and old stellar populations, and issues related to extinction (extinction law, ratio of obscuration to reddening and the like). Two methods will be used: a classical step-by-step exploration of an ensemble of standard star formation scenarii and their combinations, and an automatic inversion based on Bayesian statistics. The pros and cons of each method will be discussed.
Nov. 20 @ 15:45
Roberto Saglia (Munich/ANU)
Title: New Results on the Stellar Populations of High and Low Redshift Galaxies
The formation and evolution of galaxies is one of the key problems of modern cosmology. The observational study of the stellar populations of galaxies as a function of redshift provides important constraints to the modeling efforts of these processes. I review some recent results on the stellar populations of galaxies, starting from young, high redshift objects observed at the VLT, and finishing with old local ellipticals observed at Siding Spring.
Five starbursting high redshift (redshifts 2.2 to 3.2) galaxies are found gravitationally magnified by the cluster 1E0657-56. The equivalent width of their CIV line decreases with redshift, when compared to other UV dropout objects and to a sample of local starbursts, suggestive of metallicity evolution.
The Tully-Fisher relation (TFR) of 60 late-type galaxies of the Fors Deep Field with redshift 0.1-1 is discussed. The TFR slope is flatter than for local field galaxies, suggesting a mass-dependent luminosity evolution and the (well known) increasing importance with redshift of a population of blue, faint galaxies.
Finally, I discuss the preliminary results of a spectroscopic survey of nearby ellipticals at 8600 A. The central values of the Calcium Triplet line (CaT) are a new promising tool to study the properties of old stellar populations. Contrary to the well known Mg index, the CaT index anti-correlates with the central velocity dispersion of the galaxies.
Nov. 16 @ 11:00
Bill Harris (McMaster/ANU)
Title: Anatomy of a Galaxy: Deconstructing NGC 5128
By far the nearest giant elliptical galaxy is NGC 5128. New HST photometry for selected locations in the halo of this galaxy give us a first comprehensive look at its stellar population, and thus additional clues to its formation history.
Nov. 9 @ 11:00
Alexander Knebe (Swinburne)
Title: Warm Dark Matter: Curing the Woes of Cold Dark Matter?
Observations on galactic scales seem to be in contradiction with recent high resolution N-body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy halos.
We explored a somewhat different approach which consists of filtering the dark matter power spectrum on small scales, thereby altering the formation history of low mass objects. The physical motivation for damping these fluctuations lies in the possibility that the dark matter particles have a different nature i.e. are warm (WDM) rather than cold. We show that this leads to some interesting new results in terms of the merger history and large-scale distribution of low mass halos, as compared to the standard CDM scenario. However, WDM does not appear to be the ultimate solution, in the sense that it is not able to fully solve the CDM crisis, even though one of the main drawbacks, namely the abundance of satellites, can be remedied. Indeed, the cuspiness of the halo profiles still persists, at all redshifts, and for all halos and sub-halos that we investigated. Despite the persistence of the cuspiness problem of DM halos, WDM seems to be still worth taking seriously, as it alleviates the problems of over-abundant sub-structures in galactic halos and possibly the lack of angular momentum of simulated disk galaxies. WDM also lessens the need to invoke strong feedback to solve these problems, and may provide a natural explanation of the clustering properties and ages of dwarfs.
Nov. 8 @ 16:00
Paul Nulsen (Wollongong)
Title: AGNs, Cooling Flows and Preheating of Clusters
There is a high incidence of radio sources in the centres of cooling flow clusters. X-ray observations with Chandra and XMM-Newton show, in a rapidly growing number of cases, that radio lobes carve holes in the hot intracluster gas near to cluster centres. I will discuss what has been found about the nature of these cavities and their significance in preventing the deposition of cooled gas by cooling flows. The nuclear activity which powers the radio sources is probably fed by accretion onto the AGN of cooled gas deposited by the cooling flow. As well as preventing the deposition of substantial quantities of cooled gas, this feedback process may be responsible for the excess energy in the intracluster medium known as preheating. We do not yet know whether the main heating process is via the cavities described above, through shock heating in a rapid outburst when the radio source turns on, or some other process. The recent discovery of large-scale shocks in the hot interstellar medium of the Virgo elliptical NGC4636 shows that a rapid nuclear outburst is a significant heat source in at least this case.
Sep. 27 @ 16:00
Stuart Ryder (AAO)
Title: Circumnuclear Star Formation: Bushfires or Mexican Wave?
K-band spectra of nuclear rings in the spiral galaxies NGC 4321 (M100), NGC 2903, and NGC 7469 have been obtained with CGS4 on UKIRT. A comparison of the equivalent widths of Br-gamma emission and CO absorption with starburst models in the literature allows us to simultaneously determine the age distribution and burst decay timescales around each ring. We find that the majority of the star-forming regions are rapidly-decaying bursts with ages between 8 and 10 million years, but only NGC 4321 displays a clear age sequence. The implications of these results for the triggering and evolution of circumnuclear star formation will be discussed.
Sep. 25 @ 15:30
*** AR103 ***
Nicolas Grevesse (Liege)
Title: The Solar Chemical Composition
We review chemical abundances derived from different sources in the solar system in order to derive standard chemical composition for solar matter. This is essentially obtained from solar data as well as meteoritic data. The solar data are based on the analysis of photospheric spectra, on solar wind and solar energetic particle data, and also on solar oscillation inversion for helium. Present results for the non-volatile elements show that photospheric abundance results now perfectly agree with the very precise meteoritic data allowing us to derive a solar chemical composition which is used as a standard by astronomers and cosmochemists.
Aug. 31 @ 16:00
Tony Wong (ATNF)
Title: Radial Gas Flows in Spiral Galaxies
The possibility of radial gas flows in disk galaxies has been discussed in a number of contexts - from fueling nuclear activity and star formation to explaining the chemical evolution of galaxies. However, it is unclear whether such flows are actually observable in galaxies. I discuss how a Fourier decomposition of a Doppler velocity image can in principle be used to distinguish different types of non-circular motions - radial flow, elliptical streaming, and disk warping. Applying this technique to seven nearby spirals reveals significant departures from axisymmetric rotation in all of the galaxies, but only upper limits on pure radial flows. I then present possible indirect evidence for radial gas flows, which arises from a consideration of whether the current rates of star formation can be reconciled with the observed gas mass fractions and oxygen abundances.
Aug. 28 @ 14:00
Khalil Chamcham (Casablanca)
Title: Star Formation Thresholds and Galactic Disk Chemical Evolution
Jul. 6 @ 16:00
Bryan Gaensler (MIT)
Title: Radio Polarisation as a Probe of the Interstellar Medium
Jun. 1 @ 16:00
Mike Beasley (Swinburne)
Title: A Recipe for Globular Cluster Formation
May. 15 @ 16:00
Joss Bland-Hawthorn (AAO)
Title: The Accretion of HVCs onto the Galactic Halo
It is widely believed that the Galactic halo has built up through a process of accretion and merging of low mass structures which is still going on at a low level. Hierarchical CDM simulations predict that the Galactic halo should have many more satellites than are actually observed. The high-velocity HI clouds (HVCs) are interesting candidates except that their distances are controversial. Much of the sky is peppered with HVCs whose kinematics do not conform to orderly Galactic rotation. For the past three years, we have conducted an H-alpha + [NII] emission-line survey of HVCs at the AAT 3.9m and WHT 4.2m telescopes. Our fields were chosen from the HIPASS survey. The high spatial resolution and sampling allow us to target the high column density regions of the clouds. Our emission measures toward HVCs appear to fall into three broad categories: ~100mR, ~20mR and undetected. Interestingly, some of the non-detections fall along sight lines to compact clouds. We extend and refine the H-alpha distance scale for HVCs published by us in a series of papers since 1997. This model treats the halo ionization field in the Galaxy from the known positions of the spiral arms. For each HVC, we derive distance bounds from a model normalized by H-alpha detections towards HI complexes with known distances. The brightest HVCs (>100mR) lie inside of the Solar Circle. HVCs with emission measures ~100mR appear to lie outside of the Solar Circle out to 20 kpc radii. We suspect that the faint detections and non-detections (which fall outside the polar regions) arise from clouds at large distances (~50 kpc) which are currently accreting onto the Galactic halo.
May. 14 @ 14:00
Martin Rees (Cambridge)
Title: The End of the Dark Ages: Galaxy Formation and the First Stars
May. 10 @ 11:00
Stephen Ord (Swinburne)
Title: Measurement and Consequences of a Neutral Hydrogen Distance Limit to the Relativistic Binary Pulsar PSR J1141-6545
Apr. 23 @ 16:00
Michael Brown (NOAO)
Title: The Evolution of Galaxy Clusters
Apr. 12 @ 11:00
John Stocke (Colorado)
Title: Is this the Forest Primeval? ... HST Views the Local Lyman Alpha Forest
Detecting HI using redshifted Ly alpha absorption lines is 10^7 times more sensitive than using the 21cm line. I will review recent discoveries of HI Ly-alpha absorbers made with the Hubble Space Telescope (HST) which has allowed us a first-look at gas in intergalactic space locally (i.e., between us and the "Great Wall"). Despite its mere 2.4m aperture, these new HST observations allow the detection of absorbers as rarefied as those found using Keck at high-z (log N(HI)=12.3 cm^-2). New results that will be discussed include: the evolution of absorbers with redshift, the location of absorbers relative to galaxies (including the two-point correlation function for absorbers), the metallicity of absorbers far from galaxies and the discovery of hot, shock-heated(?) (10^{5-6} K) absorbers. The importance of VLA HI observations to discover the nearest galaxies to these absorbers will be stressed. The design and current status of the "Cosmic Origins Spectrograph" (COS) to be installed on HST in 2003 will also be discussed. COS is at least ten times more efficient than STIS for point sources and will be the most sensitive far-UV spectrograph ever installed on HST.
Apr. 3 @ 16:00
Andrew Barber (Sussex)
Title: Simulated and Observed Cosmic Shear
The detection of "cosmic shear," arising from weak gravitational lensing by the large-scale structure in the universe, provides important information about the distribution of dark matter rather than that of luminous matter. In this talk we will explore the comparisons between the recently observed cosmic shear signal and values obtained from cosmological n-body simulations. The numerical method, developed from a code for the evaluation of the shear in three dimensions, will be described and we will discuss how the results may be interpreted in terms of the cosmological parameters.
Mar. 22 @ 11:00
Kenji Bekki (UNSW)
Title: The Power of Galaxy Merging
Galaxy merging is generally considered to play a major role in the morphological transformation of galaxies, the activation of nuclear starbursts and AGNs, and the determination of the spectral energy distribution of dusty galaxies. Based upon my recent numerical simulations, I demonstrate how important it is to understand the physical processes of minor, unequal-mass, and major galaxy mergers at low and high redshift. In particular, I discuss the origin of elliptical and S0 galaxies, the formation of ultra-luminous infrared galaxies, an evolutionary link between high-redshift SCUBA sources and extremely red objects (EROs), the formation sites of young globular clusters, the evolution from nuclear starbursts to AGNs, and the formation of the Galactic stellar halo.
Mar. 8 @ 11:00
Martin Zwaan (Melbourne)
Title: HI in the Local Universe
A full assessment of the HI content and distribution of HI in the Universe is essential for understanding galaxy evolution. In this talk I will discuss the results of various HI surveys and address the following questions: Where is the HI? How is it distributed among different types of galaxies? Are there large quantities of HI locked up in dim galaxies that are easily overlooked in optical surveys? Are High Velocity Clouds the missing Galactic satellites? How does the amount of HI at the present epoch compare to that at earlier times?
Feb. 22 @ 11:00
Lionel Siess (Grenoble)
Title: Stellar Ogres
I present the results of simulations concerning the evolution of stars that have swallowed a massive planet or a brown dwarf in their interior during their giant phase. In particular I examine the case in which the planet/brown dwarf spirals in and dissipates in the inner region of the star, near the bottom of the convective envelope. I will describe the response of the stellar structure and discuss the potential observational signatures of this unusual phenomenon which include (1) the possible ejection of a shell and subsequent phase of IR emission, (2) an increase of the star rotational velocity, (3) an enhanced magnetic activity, (4) a surface enrichment in lithium and a modification of the position of the star on the horizontal branch of globular clusters. In light of these results, I will propose that the IR excess associated with Li-rich G and K giants is consistently understood in the context of the planet accretion scenario.
Jan. 25 @ 11:00
Terry Bridges (AAO)
Title: Globular Cluster Spectroscopy in the Virgo Giant Ellipticals M49 and M87
Dr. Bridges will talk about recent results obtained from spectroscopy of globular clusters in M49 and M87, specifically cluster kinematics, ages and abundances, and the galactic dark matter content. He will focus on comparing the spectroscopic properties of the metal-poor and metal-rich cluster populations.
Jan. 18 @ 11:00
Carole Mundell (Liverpool John Moores)
Title: From Kiloparsecs to Light Years: Probing the Gaseous Environment of Seyfert Galaxies