Two-Component Galaxies: Understanding the Multiple Stellar Components of Observed and Simulated Galaxies

Abstract

Galaxies are complex multi-component systems, and understanding the origins of the separate galactic components is essential to the understanding of galaxy formation. In this work, I model galaxies using just two components, but even these simple models are informative. Using observations, I study the effects of local environment on bulges and disks, especially the effects of environment on disk formation and evolution around elliptical-like bulges. Using simulations, I investigate the observable consequences of a galaxy's merger history.

In the observational portion of this work, I divide galaxies into bulge and disk components based on galaxy surface brightness profiles. Chapter 2 presents a set of multi-color bulge+disk decompositions for more than 70, 000 nearby galaxies using images from the Sloan Digital Sky Survey. I use this sample in Chapter 3 to examine the effects of environment, at fixed bulge mass, on bulges and disks. I find that the color-density relation for galaxies with an elliptical-like bulge is due entirely to the correlation between disk color and local density. There is no statistically significant correlation between local density and disk mass. However, there is evidence for a decrease in disc mass within the richest galaxy groups. Therefore, star formation quenching, which is responsible for the change in disc color, must be distinct from the processes responsible for morphological transformation. Nonetheless, the changes in disks with increased density are not sufficient to fully explain why some bulges have disks while others remain disk-less ellipticals.

In Chapter 4, I examine galaxies from hydrodynamical cosmological simulations done by Dr. Renyue Cen. Since the formation history of simulated galaxies is precisely known, I use the simulations to predict the observable consequences of a galaxy's formation history. To this end, I divide the stars in simulated galaxies into stars added in mergers and stars formed in situ. I show that the metallicity, age and orbital differences between the accreted and in situ stars lead to radial gradients in a galaxy's stellar properties, which have been observed for nearby galaxies.