Studying the Building Blocks of the Universe: the faint, low-mass galaxies

University of Minnesota Ph.D. dissertation.August 2018. Major: Astrophysics. Advisor: Claudia Scarlata. 1 computer file (PDF); xiii, 178 pages.Faint, low-mass galaxies are the next frontier in extending our understanding of how our universe evolved into its present-day state that we observe. As the ever-advancing technological prowess brings about the next generation of cutting-edge observational facilities, the limit down to which we can observe galaxies is constantly pushed to fainter fluxes and consequently, lower masses. With this new population of galaxies coming into focus, it also serves as a new set of subjects to test our models and theory of galaxy formation. While the current galaxy formation models have been widely successful at reproducing the general trends in observed properties of typical galaxies, they struggle to do so for galaxies in low mass halos. In simulations, the growth of the galaxies traces the growth of their parent dark matter halos too closely, which manifests as an over-prediction of low-mass galaxies compared to the observations. Feedback from star-formation and central black hole activity is necessary to decouple the evolution of the galaxies (made of baryonic material) from that of the dark matter halos. This is particularly critical for low-mass galaxies because of their shallow gravitational potential wells. The goal of this thesis is to understand the star-formation properties of faint, low-mass galaxies and to assemble statistically significant samples of these objects that can ultimately be used to perform more detailed follow-up studies and refine the galaxy formation models. Using deep UV imaging data obtained as part of the Hubble UltraViolet Ultra Deep Field (UVUDF) program, we measure the rest-UV luminosity functions for star-forming galaxies during the cosmic high-noon -- the peak of cosmic star-formation rate at 1.5<z<3. With samples of galaxies extending ~2 magnitudes deeper than other direct imaging studies, we definitively pin the faint-end slope for the rest-UV luminosity function at 1.5<z<3. We compare the star-formation properties of z~2 galaxies from these UV observations with results from Ha and UV+IR observations to find a lack of high SFR sources in the UV luminosity function. This can be explained by a population of dusty star-forming galaxies that are not properly accounted for by the generic relations used for dust corrections. We compute a volume-averaged UV-to-Ha ratio by abundance matching the rest-UV and Ha luminosity functions and find the observed ratio to deviate from the expectation for constant star-formation history for low-mass galaxies. We conclude that this could be due to a larger contribution from starbursting galaxies compared to the high-mass end. We next focus on the low-mass, highly star-forming galaxies characterized by the presence of strong (high EW) emission lines in their spectra and thought to be representative of the earliest galaxies in the universe. While abundant in the high redshift universe, these objects are rare at lower redshifts. In order to compile a statistically significant sample of these, we use the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) dataset, which covers 2.4 sq. deg on the Subaru-XMM Deep Field (SXDF) with deep optical, near-infrared and mid-infrared imaging. We homogenize all available imaging data on to a common astrometric reference frame to generate multi-wavelength catalog containing ~800,000 objects with photometry in 28 bands, reliable photometric redshifts, and stellar population properties. Using this multi-wavelength catalog and applying a broadband selection technique to identify galaxies with strong emission lines, we identify samples of extreme emission line galaxies and Lya-emitters over 0.06<z<0.7 and 2.4<z<3.8, respectively. The low-redshift EELGs are similar to the "Green Pea" and "blueberry" galaxies found in SDSS, albeit being much fainter and lower mass. From an initial round of spectroscopic follow-up with MMT/Hectospec, we already find one highly interesting object at z=0.069 that exhibits signatures of an exotic interstellar medium with a Balmer ratio significantly lower than the canonical Case B assumption and a particularly high O32 ratio. The EELGs in SPLASH have similar specific star-formation rates as the brighter "blueberry" galaxies from SDSS, possibly suggesting common physical processes governing their star-formation. The LAEs exhibit extreme Lya EWs that can only be caused by very recent star-formation, top-heavy IMFs and/or extremely metal poor populations. Upcoming space-based surveys such as Euclid and WFIRST-AFTA plan to unveil the nature of dark energy by performing galaxy redshift surveys observing emission line galaxies via slitless spectroscopy. These surveys will assemble an unprecedented collection of star-forming galaxies over 0.9<z