Investigating the stellar mass growth and quiescence of massive galaxies In the early universe using wide-field imaging

Stevans, Matthew Louis, Jr.
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While galaxies formed stars most actively around z=2, or ~3 Gyr after the Big Bang, when the universal star formation density in the universe peaked. By this time a population of massive galaxies had already formed 10¹¹ - 10¹² [solar mass] of stars and some had their star-formation shut off in a process known as quenching. Understanding how these massive galaxies build up their stellar mass and then quench so early in the universe is a fundamental observational test of galaxy evolution. If not obscured by dust, massive galaxies are very bright, and can be observed in the optical and infrared (IR) to probe their redshifted ultraviolet (UV) and optical emission, respectively. The UV emission is produced by newly formed O and B type stars within 100 Myrs of forming, while the rest-frame optical light is produced by stars of all type and traces the stellar mass in the galaxy. By measuring the UV and optical output of galaxies, astronomers can derive star-formation rates and stellar masses. Measuring these properties for large samples of galaxies across a wide dynamic range provides benchmarks for simulations of galaxy formation and evolution physics. The work in this dissertation focuses on completing a wide field imaging survey of galaxies with high UV star-formation rates and high stellar masses at high redshift to perform the most statistically robust census to date. In Chapter 1 we motivate measuring the UV output and the quiescent fraction of high-redshift galaxies. To measure the UV output of massive star-forming galaxies at high redshift we utilize an extensive multi-wavelength dataset assembled in the Spitzer HETDEX Exploratory Large Area Survey (SHELA) Field. The data set includes five bands of deep optical imaging from the Dark Energy Camera (DECam), deep 3.6 micron and 4.5 micron imaging for Spitzer, and J and K [subscript s] imaging for the VISTA-CFHT Stripe 82 (VICS82) Near-infrared Survey. Our extensive dataset compiled from both ground and space-based observatories is uniquely capable of studying the most actively star-forming galaxies which are often very massive galaxies residing in the rarest high-sigma density peaks of the cosmic web. In Chapter 2 we study the bright end of the z=4 galaxy UV luminosity distribution or luminosity function by fitting the spectral energy distributions (SEDs) of the galaxies in our photometric data with Stellar Population Synthesis (SPS) models to measure the galaxies' redshifts and UV luminosity. In addition to measuring the bright end of the galaxy luminosity function, we had the unanticipated result of measuring the faint end of the z=4 active galactic nuclei (AGN) UV luminosity function, which has implications on the contribution of AGNs during the end of the reionization era. We compare our observed galaxy luminosity function to luminosity functions predicted by semi-analytical models (SAMs) with different prescriptions for star formation physics, such as the density of neutral hydrogen. We find our observations are consistent with predictions that galaxies at z=3-4 form stars more efficiently than at lower redshifts due to shorter neutral hydrogen depletion times. In Chapter 3, we measure the fraction of massive (M [subscript *] > 10¹¹ [solar mass] galaxies at z=3-5 in the largest volume to date. To do this we produce a K [subscript s] -selected catalog by combining deep K [subscript s] -band imaging from the NEWFIRM HETDEX survey (NHS), which we obtain, reduce, and catalog. We select quiescent galaxies by performing SED-fitting with SPS models to measure their redshifts, SFRs, and stellar masses. We define quiescent galaxies as having a specific SFR (sSFR; sSFR = SFR / stellar mass) < 10⁻¹¹ yr⁻¹. We measure a quiescent fraction of 10+/-4% among these massive galaxies with reduced errors due to cosmic variance and Poisson noise, which is consistent with the next largest sample of this galaxy population. We find that while the population of massive galaxies at z=3-5 is dominated by star-forming galaxies, a significant fraction have quenched, which suggests there exists a population of massive galaxies at even higher redshifts that have rapidly formed their stars and quenched in less than ~1.5 Gyr since the Big Bang. In Chapter 4 we present the outlook for studying the formation and evolution of massive quiescent galaxies in the early universe using current and future observatories