Constraints on the star formation histories of galaxies from z ∼ 1 to 0

We present a new method to estimate the average star formation rate per unit stellar mass (SSFR) of a stacked population of galaxies. We combine the spectra of 600-1000 galaxies with similar stellar masses and parametrize the star formation history of this stacked population using a set of exponentially declining functions. The strength of the hydrogen Balmer absorption-line series in the rest-frame wavelength range 3750-4150 angstrom is used to constrain the SSFR by comparing with a library of models generated using the BC03 stellar population code. Our method, based on a principal component analysis, can be applied in a consistent way to spectra drawn from local galaxy surveys and from surveys at z similar to 1, and is only weakly influenced by attenuation due to dust. We apply our method to galaxy samples drawn from Sloan Digital Sky Survey and DEEP2 to study the mass-dependent growth of galaxies from z similar to 1 to 0. We find that (i) high-mass galaxies have lower SSFRs than low-mass galaxies and (ii) the average SSFR has decreased from z=1 to 0 by a factor of similar to 3-4, independent of galaxy mass. Additionally, at z similar to 1, our average SSFRs are a factor of 2-2.5 lower than those derived from multiwavelength photometry using similar data sets. We then compute the average time [in units of the Hubble time, t(H)(z)] needed by galaxies of a given mass to form their stars at their current rate. At both z = 0 and 1, this time-scale decreases strongly with stellar mass from values close to unity for galaxies with masses similar to 10(10) M(circle dot), to more than 10 for galaxies more massive than 10(11) M(circle dot). Our results are in good agreement with models in which active galactic nuclei feedback is more efficient at preventing gas from cooling and forming stars in high-mass galaxies.