David Fisher’s Research:

Multiwavelength Studies of Galaxies


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.

Pseudobulges and Classical Bulges in Disk Galaxies

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

You can look at my publication list here.

I am 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.

Infrared and Millimeter Techniques

As a member of the CARMA community I work 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.

The overriding goal in my research is to address the question, “Why do galaxies look the way they do?” Its a very simple question, that turns out to have a very complicated answer. To answer this I frequently employ a variety of methods. I have done a great deal of work with bulge-disk decompositions of optical and near-IR data. I measured stellar population properties using HST, 2MASS and SDSS  data (most frequently to measure accurate stellar mass-to-light ratios). I have derived and used star-formation rate indicators using GALEX and Spitzer MIPS data. As well I have worked with millimeter and sub-mm data to measure gas and dust properties.

The Structure of Elliptical Galaxies

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 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)   http://arxiv.org/abs/0810.1681shapeimage_6_link_0

Dust & Star Formation at Extremely Low Metallicities

I Zw 18 is the lowest metallicity galaxy 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. Our group is trying to characterize properties like the dust mass and molecular gas content in order to test such theories. 

Left is an image of I Zw 18 from Hubble Heritage site.