The Fossil Record of Star Formation from Galaxy Spectra

Abstract

In this thesis I present work using the MOPED algorithm to extract in a non-parametric fashion star formation histories and galaxy masses from the spectra of galaxies in the Sloan Digital Sky Survey. The recovered parameters for all galaxies are combined to give insight into the processes of star and galaxy formation on both individual galaxy and cosmic scales. The MOPED algorithm allows use of the entire spectral range, rather than concentrating on specific features, and can be used to estimate the complete star formation history without prior assumptions about its form. By combining the star formation histories of 96,545 galaxies in the redshift range 0 < z < 0:34 the cosmic star formation rate is determined from the present day to z ~ 6. The results show that the peak of star formation occurred at z ~ 0:6, and that 26% of the mass of stars in the present-day Universe was formed at z ~ 2. The average metallicity rises from Z/Z= 0:44 at high redshift to a peak of 0:8 at z ~ 1 before declining to a level around 0.25 atthe present day. Although the peak in star formation is more recent than previously thought, the sample used includes galaxies with a range of masses not accessible to traditional studies, down to a limit of L ~ 2 x 10-3L*. By cutting the sample into ranges of mass it can be seen that the redshift at which starformation activity peaks is an essentially monotonically increasing function of final stellar mass. The time of the peak in star formation ranges from z > 2 for the highest mass galaxies (MS < 1012M) to z ~ 0:2 for the lowest (MS < 1010M). A typical L* galaxy appears to have its peak at around z » 0:8. These differences in star formation with mass reconcile the redshift of the peak found in this work with the previous estimates, generally deep surveys only probe the SFR of galaxies with MS < ML*. The stellar mass calculated using the reconstructed spectra eliminates contamination from either emission lines or AGN components. Using these masses it is possible to construct the mass function for the stellar mass component of galaxies which give excellent agreement with previous works, but extend their range by more than two decades in mass to 10 7.5 < Ms/h-2M < 1012. I present both a standard Schechter fit and a fit modified to include an extra, high-mass contribution, possibly from cluster cD galaxies. The Schechter fit parameters are phi* = (7:8 +/- 0:1) £ 10-3h3Mpc-3, M* = (7.64 +/- 0.09) x 10*10h-2M and alpha = -1.159 +/- 0.008. The sample also yields an estimate for the contribution from baryons in stars to the critical density of omega b*h = (2.39+/-0.08)x10-3, in good agreement with other indicators. No evolution of the mass function in the redshift range 0:05 < z < 0:34 is apparent, indicating that almost all stars were already formed at z » 0:34 with little or no star formation activity since then and that the evolution seen in the luminosity function must be largely due to stellar fading. The star formation history can be interpreted as a measure of how gas was transformed into stars as a function of time and stellar mass: the Baryonic Conversion Tree (BCT). There is a clear correlation between early star formation activity and present-day stellar mass: the more massive galaxies have formed about 80% of their stars at z > 1, while for the less massive ones the value is only about 20%. Comparing the BCT to the dark matter merger tree indicates that star formation efficiency at z > 1 had to be high (as much as 10%) in galaxies with present-day stellar mass larger than 2 x 10*11M, if this early star formation occurred in the main progenitor. The LCDM paradigm can accommodate a large number of red objects; it is the high efficiency in the conversion from gas to stars that needs to be explained. On the other hand, in galaxies with present-day stellar mass less than 10*11M, efficient star formation seems to have been triggered at z ~ 0:2. This work shows that there is a characteristic mass (M » 10*10M) for feedback efficiency (or lack of star formation). For galaxies with masses lower than this, feedback (or star formation suppression) is very efficient while for higher masses it is not. The BCT, determined here for the first time, should be an important observable with which to confront theoretical models of galaxy formation.