Title: The Role of Cold Gas in Massive Galaxy Evolution
Description: The cold gas content of a galaxy reflects its past assembly history as well as its potential for future star formation. It has been shown to be tied to a galaxy's morphology, current star formation rate, and environment. In combination with large surveys at optical and ultraviolet wavelengths, measurements of the cold gas in galaxies provide new ways of understanding and examining the complex relationship between cold gas and derived quantities related to structural and star-forming properties. In particular, measurements of the cold gas content of galaxies can help inform the interpretation of global scaling relations in the local population of galaxies that cannot be fully understood without knowledge of the gas content of galaxies. However, the long integration times required to detect neutral hydrogen gas (HI) and CO make it difficult to detect low levels of cold gas in large numbers of galaxies. Where the presence of cold gas is assumed to be important but observations are not available, we can sometimes make assumptions to estimate the precise role of cold gas.

With the advent of large surveys measuring cold gas in representative samples of galaxies, we have been able to study the cold gas in galaxies in a statistical way that rivals the methods by which we can study other properties of galaxies. In this thesis we use measurements from GALEX, SDSS, and the GALEX Arecibo SDSS Survey (GASS; Catinella et al. 2010) to quantitatively describe the distribution of cold gas in massive galaxies using sophisticated techinques that had previously only been applied to quantities derived from optical and ultraviolet observations. We also show how we can use distribution functions derived from large surveys to identify galaxies in distinct evolutionary phases that can shed light on the processes of galaxy evolution in general, and more specifically, about the crucial role HI plays in driving the evolution of massive galaxies. In Chapter 2 we design a classification scheme to identify galaxies with unexpected star formation in their outer disks (extended ultraviolet, or XUV, disks) and to extend the known sample of such objects out to moderate (z~0.05) redshifts.

We find that 20% of galaxies in the most nearby portion of the sample exhibit XUV-disks and that XUV-disks are surprisingly common in massive, bulge-dominated galaxies. From our large, unbiased sample of galaxies we derive the space density of XUV-disks in the local universe. With this derived space density, and based on the assumption that XUV-disks must form in extended cold gas disks, we estimate the cold gas accretion rate onto XUV-disks in the local universe. In Chapter 3 we derive the bivarate HI-stellar mass function for massive galaxies, which is a crucial tool for constraining simulations. We test six different parameterizations of the distribution function and we also examine how the shape of the distribution function depends on star formation rate. We find that the location of the peak in the distribution function does not depend strongly on stellar mass or star formation rate but that the slope of the distribution function at low masses does. We also discuss how physical processes drive the shape of the bivariate HI-stellar mass function.

Finally, in Chapter 4 we demonstrate the utility of scaling relations derived from large datasets by using the gas fraction scaling relation to select an anomalous sample of massive HI-rich galaxies with surprisingly low levels of star formation. We obtain HI imaging of these galaxies to ascertain why so much of their cold gas content is not participating in star formation. All of the galaxies we observe exhibit extended HI disks whose gas surface densities are below the threshold required for star formation. Since this type of galaxy is most prevalent at stellar masses above the transition mass noted in Kauffmann et al. (2003), it is possible that the processes inhibiting star formation in these galaxies contribute to the change in star-forming properties above the transition mass.