UC Berkeley

Over the past decade, large photometric surveys, and more recently spectroscopic follow up, have elucidated a clearer picture of galaxy evolution since the epoch of peak star formation at z = 1-3. However, despite these advancements, low mass galaxies, which exhibit much higher number densities than their $Log M_{\star}>9.5$M$\odot$ counterparts, have remained challenging to follow up observationally with current instruments given their faintness and sizes. Despite challenging systematics, gravitational lensing induced magnification and flux amplification is one path towards studying the faint high redshift Universe. Another complimentary approach lies in utilizing the stellar fossil record to turn back the clock on local resolved stars and place constraints on their progenitors. Advances in magnification maps and deep photometry from the Hubble Frontier Fields (HFF) survey, as well as improvement in stellar models and distance measurements, enable us to bridge between the near and far fields to study star formation in low mass galaxies across cosmic time.

In this dissertation, I use rest-frame optical spectroscopy from the MOSFIRE Deep Evolution Field (MOSDEF) survey to investigate the star formation histories (SFHs) of different galaxy types, ranging from actively star-forming to quiescent at $1.4\leq~z\leq2.6$. SFHs are constrained using stellar continuum spectroscopy, specifically through a combination of Balmer absorption lines, the 4000$\,\Angstrom$ break, and the equivalent width of the H$\alpha$ emission line. I use Spectral Energy Distribution (SED) modeling to constrain the average SFHs for five galaxy types including quiescent, green valley, and post-starburst galaxies (transitional galaxies). I find that quiescent and transitional galaxies in the MOSDEF sample are dominated by an SFH with an average star-formation timescale of $\tau \sim 0.1-0.2$~Gyr, in contrast to galaxies in the low-redshift Universe which form their stars over a more extended time period ($\tau>1$~Gyr) on average. Finally, I compare the average properties of the galaxies in the transitional bins to investigate possible paths to quiescence, and speculate on the viability of a dusty post-starburst phase.

I then utilize the the stellar fossil record and stellar population synthesis modeling to reconstruct the evolution of rest-frame ultra-violet (UV) luminosities of the most massive Milky Way dwarf spheroidal satellite galaxy, Fornax, and its five globular clusters (GCs) across redshift. I find that (1) Fornax's (proto-)GCs can generate $10-100$ times more UV flux than the field population, despite comprising $<\sim 5\%$ of the stellar mass at the relevant redshifts; (2) due to their respective surface brightnesses, it is more likely that faint, compact sources in the HFF are GCs hosted by faint galaxies, than faint galaxies themselves. This may significantly complicate the construction of a galaxy UV luminosity function at $z>3$. (3) GC formation can introduce order-of-magnitude errors in abundance matching. I also find that some compact HFF objects are consistent with the reconstructed properties of Fornax's GCs at the same redshifts (e.g., surface brightness, star formation rate), suggesting we may already have detected proto-GCs in the early Universe.

Finally, I present results from a Keck/MOSFIRE survey of 39 low mass galaxies in the HFF. I first discuss the sub-structure around the progenitor of a Milky Way-mass galaxy in the Hubble Frontier Fields (HFF). Specifically, I study an $r_e = 40^{+70}_{-30}$pc, $M_{\star} \sim 10^{8.2} M_{\odot}$ rest-frame UV luminous ``clump'' at a projected distance of $\sim$100~pc from a $M_{\star} \sim 10^{9.8}$M$_{\odot}$ galaxy at $z = 2.36$ with a magnification $\mu = 5.21$. I measure the star formation history of the clump and galaxy by jointly modeling the broadband SED from HFF photometry and H$\alpha$ from MOSFIRE spectroscopy, and explore methodology to constrain whether it formed $in-situ$ or presents a $1:40$ stellar mass ratio accretion event.

I then explore the recent star formation histories and kinematics of the rest of the sample which consists of: $7 \le \log M_{\star} / M_{\odot} \le 10.2$ moderately gravitationally lensed ($\mu<20$), star-forming galaxies at $1.8 \le z \le 3.2$. I find that \lha \ star formation rates (SFRs) exceed UV SFRs by factors of $\sim3-10$, indicative of highly bursty star formation. Galaxy size ($r_e$) is a strong function of decreasing \B\ and \sigha \ (but not \siguv). This suggests that $r_e$ is likely set by the ratio of instantaneous ($<10$Myr) to recent star ($<100$Myr) formation in these low-mass galaxies. I compare integrated velocity dispersions ($\sigma$) with specific SFRs and find a strong correlation at fixed stellar mass. This is consistent with scenarios in which stellar feedback is driving changes to the potential of low-mass galaxies. Finally, I compare the dynamical masses of my galaxy sample with their inferred total baryonic masses using the Kennicutt-Schmidt (KS) relation and find that my inferred baryonic mass is un-physical, even when size underestimates are considered, suggesting that the galaxies in my sample are gas poor compared to local ones.