Star formation and metallicity history of the SDSS galaxy survey: unlocking the fossil record

Using MOPED, we determine non-parametrically the star formation and metallicity history of over 37 000 high-quality galaxy spectra from the Sloan Digital Sky Survey (SDSS) early data release. We use the entire spectral range, rather than concentrating on specific features, and we estimate the complete star formation history without prior assumptions concerning its form (by constructing so-called 'population boxes'). The main results of this initial study are that the star formation rate in SDSS galaxies has been in decline for 6 Gyr; a metallicity distribution for star-forming gas, which is peaked 3 Gyr at approximately solar metallicity, inconsistent with closed-box models, but consistent with infall models. We also determine the infall rate of gas in SDSS and show that it has been significant for the last 3 Gyr. We investigate errors using a Monte Carlo Markov chain algorithm. Furthermore, we demonstrate that recovering star formation and metallicity histories for such a large sample becomes intractable without data compression methods, particularly the exploration of the likelihood surface. By exploring the whole likelihood surface, we show that age-metallicity degeneracies are not as severe as using only a few spectral features. We find that 65 per cent of galaxies contain a significant old population (with an age of at least 8 Gyr), including recent starburst galaxies, and that over 97 per cent have some stars older than 2 Gyr. It is the first time that a complete star formation and metallicity history, without restrictive assumptions concerning its form have been derived for such a large data set of integrated stellar populations, and the first time that the past star formation history has been determined from the fossil record of the present-day spectra of galaxies.