Determining Properties of LEGA-C Galaxies through Spectral Star-formation History Reconstruction

Over the past decade, photometric and spectroscopic surveys have enabled us to obtain an integrated view of galaxy evolution. We have measured the cosmic star formation history, and we know that about half of the stars we observe formed before the Universe was half of its current age. However, crucial knowledge of individual galaxy evolution has been limited because the detailed stellar population properties that we know about galaxies, such as ages, metallicities and kinematics, have mostly been obtained from nearby galaxies, which contain mostly old stellar populations. To advance our understanding of galaxy evolution, measurements have to be extended to more distant galaxies, which have a wider variation in age and star formation (SF) activity. In this thesis, I investigate the stellar population properties of a galaxy population at half the Universe’s current age (redshift z ~ 1), using high quality, high resolution spectra from the (recent) LEGA-C Survey. I present an algorithm that uses full-spectrum fitting to reconstruct the star formation histories (SFHs) of LEGA-C galaxies. First, I investigate the dependance of individual SFHs on stellar mass, stellar velocity dispersion and SF activity. The individual SFHs of high-mass (velocity dispersion) quiescent and star-forming populations are increasingly divergent towards lower redshifts, which indicates a strong correlation between current SF activity and past SF activity. Second, I trace the stellar mass evolution of LEGA-C galaxies between redshifts z ~ 1 and z = 3 (when the Universe was a sixth of its age). Galaxies that are similar in stellar mass at one redshift, have diverse evolutionary paths that lead to a wide range of stellar masses at another redshift, whether they are being traced backward or forward in time. Last, I investigate rejuvenation in quiescent galaxies, that is, when galaxies that are quiescent, re-ignite their SF before becoming quiescent again. Although, rejuvenation events do not contribute significantly to the growth of quiescent galaxies, a majority of rejuvenated galaxies lie in the ‘green valley’, where galaxies are thought to be in the (one-way) transition phase from the blue cloud to the red sequence. Reconstructing SFHs has allowed us to trace the individual pathways along which galaxies evolve, and investigate the physical processes that drive them. Measurements from this work, as well as other LEGA-C studies, will be important for future studies at even higher redshifts because they will act as a benchmark to connect populations at higher redshifts to the nearby Universe