UC Santa Cruz

The processes that fuel and quench star formation in galaxies are expected to leave imprints on their structure. Moreover, these imprints can serve as signposts to identify galaxies at various stages of their evolution. In this dissertation, we describe our efforts to characterize changes in galaxy structure (1) as star formation is going out, (2) after star formation has ostensibly quenched, and (3) during active star formation, with the goal of elucidating the relevant processes that regulate star formation in each case.

In Chapter 2, we use a local (z~0) sample of galaxies drawn from the Sloan Digital Sky Survey (SDSS) to investigate the first two cases. We find that, as galaxies quench, their outer stellar mass density profiles remain essentially constant. However, their inner stellar mass density (within 1 kpc) increases, even while galaxies are still star-forming, until it reaches a mass-dependent threshold, at which point quenching can occur. The existence of this threshold indicates that quenching is connected with processes that grow bulges.

In Chapter 3, we then study the morphologies and color profiles of 19 z~0 early-type galaxies located in the green valley. Combining high-resolution Hubble ultraviolet images with SDSS optical photometry, we find that these objects harbor low-level star formation that is clearly detectable in the ultraviolet. Moreover, the recently formed stars are distributed in symmetric rings that often span the entire optical extent of the galaxy. The presence of an old underlying population in their outer parts suggests that star formation in these galaxies is either gradually fading out or possibly rejuvenated by smooth accretion from the intergalactic medium. Such galaxies comprise ~13% of green valley galaxies of similar mass and color, and they may linger in the green valley for several Gyr.

Finally, in Chapter 4, we focus on the evolution of star-forming galaxies since z=2.5, leveraging the rich multi-wavelength datasets produced by the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). By finely dissecting our sample into narrow mass and redshift slices, we are able to uncover several striking trends, including evidence for mass-dependent evolution in terms of disk formation and the buildup of metals and dust, i.e., massive galaxies are more evolved at all epochs. However, at fixed mass, no correlation between structure and specific star-formation rate is seen for galaxies on the star-forming "main sequence." This behavior persists at all redshifts, despite the considerable diversity in size and shape among star-forming galaxies. This suggests that, on the main sequence, star formation is regulated primarily by external factors, e.g., halo accretion rate, rather than the internal structure of the galaxy.