How Do Bulges Form In Disk Galaxies?

Over the last decade our understanding of bulges in nearby disk galaxies has dramatically changed. My main research is to try to understand what physical mechanisms are responsible for the higher surface brightness in the centers of disk galaxies (that we typically refer to as the bulge). We now know that at least two kinds of bulges exist in nearby disk galaxies. “Classical Bulges” are those bulges with properties similar to elliptical galaxies (steep stellar surface brightness profiles, quiescent and low density ISM, hot stellar dynamics). “Pseudobulges” are those bulges with properties more similar to galactic disks (nearly exponential surface brightness profiles, actively forming stars, rotating stellar dynamics). I have shown that surface brightness profile shape is a fairly robust means of identifying pseudobulges and classical bulges (Fisher & Drory 2008 and 2010). Also I have shown that pseudobulges are forming higher amounts of stars per unit stellar luminosity (Fisher 2006 and Fisher et al. 2009).

ADS Listing for David Fisher
Astro-PH Listing for David Fisher

Current Work

I am currently using CARMA data to measure the radial distribution of molecular gas in bulge-disk galaxies. I also use GALEX and Spitzer data to measure formation rates, and near infrared data to measure radial distribution of stellar mass. Constraining the star formation and molecular gas content of bulges allows us to determine how much evolution has occurred in recent epochs. Using data from CARMA I can estimate what fraction of the baryonic mass is younger than a couple gigayears. I work to place this in the context of galaxy evolution by studying links between the amounts of recent evolution in bulges with galaxy properties (such as total stellar mass or bulge-to-total stellar light ratio). I am also involved in a survey to map the stellar kinematics of pseudobulges with a new IFU capable of measuring disk velocity dispersions, and recently I have begun working with a group attempting the first realistic simulations of bulge-disk galaxies in a cosmologically motivated environment.

Working Hypothesis

Though no detailed theory exists to explain the formation of bulges, a working hypothesis is that pseudobulges form through slow evolution of disk gas, in response to torques by disk gas (see Kormendy & Kennicutt 2004 for an in depth review of this process). Connections between bulge and disk properties observed only for pseudobulges (Fisher & Drory 2008, Fisher et al. 2009) support this hypothesis. The more common prescription to forming bulge mass is through mergers, because they are fast and violently relaxes stars; it seems likely that classical bulges form this way. The different timescales for these processes means that measuring the recent evolution of bulges is a good way to determine how they may have formed.
In Fisher & Drory (2011) we estimate the frequency of each bulge type. We find that roughly 2/3 of bulges in the local 11 Mpc are pseudobulges. This is shown in the figure. Thus implying that understanding how pseudobulges means understanding how the most well studied bulges formed their mass. I am interested in continuing similar studies in different environments.

Survey Towards Infrared-bright Nearby Galaxies

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. For more information, click here

[CII] Emission From Nearby Galaxies

I am working with a graduate student in the LMA on understanding the emission of [CII] in from nearby galaxies. [CII] is the strongest cooling line in the interstellar medium of nearby galaxies. It is therefore a useful probe of the state of the ISM. The [CII] emission is triggered by UV photons, and is thus a good candidate for use as a star formation rate indicator and also a probe of the local radiation field.