Galaxy formation as a function of redshift and environmentD. Thomas, K. Schawinski, J. Seok-Joo, C. Maraston, M. Sarzi, S. Kaviraj, S. Yi
I discuss how astro-archaeology, the analysis of stellar populations in the local and moderately redshifted universe, can constrain galaxy formation by providing very direct probes for the evolutionary histories of galaxies and hence their formation epochs. I present new results obtained from the analysis of 20,000 elliptical galaxies drawn from the Sloan Digital Sky Survey. We confirm and statistically strengthen earlier results that mean ages, metallicities and alpha/Fe element ratios of ellipticals correlate tightly with galaxy mass. Most surprisingly and differently from previous work, however, these scaling relations turn out to be independent of environmental density for the bulk of the population. At the low-mass end some fraction of the galaxies show signs of recent star formation on top of the general age-mass relationship. It is only this fraction that reveals strong dependence with environement increasing from only a few per cent in densest clusters to about 25 per cent in the most isolated regions of the universe. I further show that these relationships have not evolved in zero point or slope at least during the last third of the universe's lifetime. These results imply a strong link between the histories of star formation and mass assembly of massive galaxies, which poses a key challenge to theory of galaxy formation.