David B Fisher : Astronomer

dfisher "at" swin.edu.au
HST Heritage image of NGC 628
Keck firing the laser AO system while I'm using it.


I'm a lecturer at Swinburne University in Melbourne, Australia. My actual position is Director's Research Fellow. I received my PhD from the University of Texas in 2010. My advisor was John Kormendy.

In general I study the properties of galaxies in the context of galaxy evolution. My main focus these days (2016) is a sample of extremely gas rich, turbulent disk galaxies at low redshift. These galaxies allow us to probe a unique environment in which star burst conditions are driven with out mergers. In the past, most of my research is in trying to understand how bulges form, and why the dichotomy of bulges (into pseudobulges and classical bulges) exists. I also am very interested in studying star formation and gas properties of nearby galaxies. I have also done work on the structure of elliptical galaxies. I regularly work with multiwavelength data sets including UV, optical, near infrared, IR and sub-mm.

For a more detailed description of what I do, click the topics listed below. You can also check out my publications on ADS.

Research Multiwavelength Studies of Galaxies

I have worked with variety of observational methods. I have done a great deal of work with bulge-disk decompositions of optical and near-IR data, and I developed a methods to combine data sets to create more accurate bulge-disk decompositions. I measured stellar population properties using HST, 2MASS and SDSS data (most frequently to measure accurate stellar mass-to-light ratios). I have generated my own correction to star formation rate indicators that accounts for the contribution of old stars to infrared and UV data. I work with AO enabled systems (these days on Keck), and I employ a number of techniques to push resolution as much as possible (sometimes doing better than HST). I have worked in detail with both IR (Spitzer and Herschel) and millimeter CARMA and ALMA techniques. While working at CARMA I maintained the pointing system of the telescope array.

Galaxies in the high redshift (z=1-3) Universe were somewhat different than local galaxies. At high redshift galaxies were gas rich and forming impressive amounts of new stars. This star formation occurred in giant complexes of molecular gas roughly 1,000 times larger than the molecular clouds found in our galaxy. Very little is known about this extreme star formation, since its almost exclusively observed in distant galaxies which are faint and small.

The DYNAMO survey is a sample of galaxies that show similar properties to high-z turbulent, clumpy disks, but the DYNAMO galaxies are much closer. You can see in the figure I show that DYNAMO galaxies bear a remarkable likeness to the simulations designed to match the conditions of the early Universe. Using the DYNAMO sample we can study the properties of the star forming clumps in much greater detail. Though quite rare, these local galaxies provide a window into a type of star formation that dominated the Universe from z=1-3. I have recently published the analysis of our HST maps of ionized gas (see Fisher et al 2016), and I am leading the effort to collect information on the molecular gas properties of DYNAMO galaxies, including data from NOEMA and ALMA. Right now (late 2016) in DYNAMO we are finding a lot of fun new results so keep an eye out for new works.

DYNAMO Publications: Fisher et al. (2016) , Obreschkow et al.(2015), Bassett et al.(2015), Fisher et al.(2014)

I Zw 18 from Hubble Heritage site.

I Zw 18 is the among the lowest metallicity galaxies in the nearby Universe. It therefore provides us with a test case for theories of star formation that depend on galaxy metallicity. Furthermore low metallicity galaxies give us nearby analogues to the type of star formation that occurs in the very early Universe, when the Universe had not yet produced very many metals. Using deep photometry from the Herschel Space Observatory I lead a recent group to measure the dust mass in this galaxy. We find that the dust mass is 100x smaller than typical models predict! This implies that (1) models of the interstellar medium in low metallicity galaxies produce too much dust, and (2) it will be harder than we thought to measure the interstellar medium of the earliest galaxies in the Universe.

This work was recently published in Nature. I was interviewed for the weekly ABC radio program "Star Stuff" about this result.  You can listen here (first 10 minutes is about I Zw 18).

Read more: Fisher et al. (2014) , Herrera-Camus, Fisher et al. (2013)

Pseudobulges are the most common type
of bulge in the local Universe.

Over the past decade we have learned that there seems to be two types of bulges in disk galaxies. One type of bulge is similar to elliptical galaxies. These are called "classical bulges." Classical bulges are featureless non-star forming systems, that fit the traditional picture of how bulges are made. The other type of bulge (called a "pseudobulge") has properties more similar to disk galaxies. Pseudobulges are star-forming and gas rich, they have structures like nuclear spirals that are not typical of elliptical galaxies. The look and act like disks.

It may be that this dichotomy arises because there are two ways to make a bulge. First, its plausible that classical bulges are formed through mergers (since there features resemble the end products of merger simulations). However, there is a chance that pseudobulges are not formed through mergers. Indeed, some have argued that pseudobulges form directly out of disk material through "secular evolution." Most models of galaxy evolution only build bulges through mergers. In Fisher & Drory (2011) we showed that in the local Universe most bulges are pseudobulges. The figure on the right shows that pseudobulges dominate intermediate mass galaxies, and classical bulges only at very high masses. So it may be that models of galaxy evolution are not describing the properties of nearby disk galaxies.

Read more: Fisher et al. (2013), Fabricius et al. (2012), Fisher & Drory (2011) ,Fisher & Drory (2010), Fisher et al. (2009), Fisher & Drory (2008), Drory & Fisher (2007), Fisher (2006)

I am loosely involved with a number of projects studying the interstellar medium of nearby galaxies. I am part of a project to use ALMA data to analyze interstellar medium of nearby starbursting galaxy NGC 253. We recently had a Nature paper (Bolatto et al. 2013) that makes the first measurement of a molecular gas outflow from a star forming region.

I was part of a group (KINGFISH) that used Herschel data to callibrate the [CII] gas emission line as a star formation rate indicator. [CII] is the dominate cooling line in the ISM and is very useful for studying high redshift galaxies (as it gets redshifted into the ALMA bands).

I was also a member of the STING collaboration, which is a survey of nearby galaxies using CARMA. The survey is primarily to study the molecular gas of 27 nearby galaxies covering a significant range of stellar mass, star formation rate and galaxy morphology. STING maps CO(1-0) emission of these galaxies out to roughly the optical radius, giving a larger dynamic range of stellar density and radius than similar surveys. The picture shows false color images of the IR emission of STING galaxies. Projects in the STING collaboration include  studied the gas consumption timescale of star forming regions, measuring the dust chemistry of different regions, as well the STING galaxies are included in my study of the molecular gas content of bulges.

I'm also a member of the VENGA survey which used integral field unit spectroscopy to study the interstellar medium and star formation in nearby disk galaxies.

Read more:  Bolatto et al. 2013, Fisher et al. (2013), Wong et al. (2013), Raman et al. (2011)

There is a well known dichotomy in properties of elliptical galaxies. As a graduate student I did a significant amount of work on the structure of elliptical galaxies in the Virgo cluster. We showed that the shape of surface brightness profiles of elliptical galaxies is part of this dichotomy. Bright slow-rotating elliptical galaxies have higher Sersic index than low-luminosity ellipticals. Furthermore, we showed that there is an excess in the central surface brightness of low luminosity E galaxies. We show that these properties are consistent with simulations of mergers that involve larger gas fractions.

We therefore interpret our results to be indicative that low luminosity elliptical galaxies are likely the product of gas rich mergers of disk galaxies, and higher luminosity elliptical galaxies are likely the result of lower gas fraction mergers (possibly of elliptical galaxies). The signatures of this can be tied to details of the surface brightness profile.

Read more: Kormendy, Fisher, Bender, Cornell (2010) Kormendy et al. (2005)

Currently at Swinburne I am involved in a number of projects using Keck telescopes . Personally I really enjoy working with Adaptive Optics systems. Adaptive optics allows us to observe very high spatial resolution, however care and attention to detail is required to account for the highly variable near-infrared sky. Also the point-spread function of adaptive optics is much more complex than standard optical systems, and requires skill in proper characterization. I am always looking interested in ways to push resolution of AO systems.

As a member of the CARMA community I worked on maintaining the CARMA system. Particularly I help maintain the pointing of CARMA telescopes. Also, I am working at applying interferometry techniques to mid-IR data to combine Herschel and Spitzer MIPS data to construct higher quality data than either image by itself.