The history of star-forming galaxies in the Sloan Digital Sky Survey

Abstract

We study the evolution of 82302 star-forming (SF) galaxies from the SDSS. Our\nmain goals are to explore new ways of handling star formation histories (SFH)\nobtained with our publicly available spectral synthesis code STARLIGHT, and\napply them to investigate how SFHs vary as a function of nebular metallicity\n(Zneb). Our main results are: (1) A conventional correlation analysis shows how\nglobal properties such as luminosity, mass, dust content, mean stellar\nmetallicity and mean stellar age relate to Zneb. (2) We present a simple\nformalism which compresses the results of the synthesis into time-dependent\nstar formation rates (SFR) and mass assembly histories. (3) The current SFR\nderived from the population synthesis and that from H-alpha are shown to agree\nwithin a factor of two. Thus we now have a way to estimate SFR in AGN hosts,\nwhere the H-alpha method cannot be applied. (4) Fully time-dependent SFHs are\nderived for all galaxies and averaged over six Zneb bins spanning the entire SF\nwing in the [OIII]/H-beta X [NII]/H-alpha diagram. (5) We find that SFHs vary\nsystematically along the SF sequence, such that low-Zneb systems evolve slower\nand are currently forming stars at a higher relative rate. (6) At any given\ntime, the distribution of specific SFRs for galaxies within a Zneb-bin is broad\nand roughly log-normal. (7) The same results are found grouping galaxies in\nstellar mass (M*) or surface mass density (S*) bins. (8) The overall pattern of\nSFHs as a function of Zneb, M* or S* is robust against changes in selection\ncriteria, choice of evolutionary synthesis models for the spectral fits, and\ndifferential extinction effects. (Abridged)\n