Specific Star Formation Rates to Redshift 1.5

Abstract

We present a study to determine how star formation contributes to galaxy growth since z = 1.5 over five decades in galaxy stellar mass. We investigate the specific star formation rate (SSFR; star formation rate [SFR] per unit galaxy stellar mass) as a function of galaxy stellar mass and redshift. A sample of 175 K-band-selected galaxies from the MUnich Near-Infrared Cluster Survey spectroscopic data set provides intermediate- to high-mass galaxies (mostly M_* ≥ 10¹⁰ M_☉) to z = 1. The FORS Deep Field provides 168 low-mass galaxies (mostly M_* ≤ 10¹⁰ M_☉) to z = 1.5. We use a Sloan Digital Sky Survey galaxy sample to test the compatibility of our results with data drawn from a larger volume. We find that at all redshifts, the SSFR decreases with increasing galaxy stellar mass, suggesting that star formation contributes more to the growth of low-mass galaxies than to the growth of high-mass galaxies and that high-mass galaxies formed the bulk of their stellar content before z = 1. At each epoch, we find a ridge in SSFR versus stellar mass that is parallel to lines of constant SFR and evolves independently of galaxy stellar mass to a particular turnover mass. Galaxies above this turnover mass show a sharp decrease in the SFR compared to the average at each epoch, and the turnover mass increases with redshift. The SFR along the SSFR ridge decreases by roughly a factor of 10, from 10 M_☉ yr^-1 at z = 1.5 to 1 M_☉ yr^-1 at z = 0. High-mass galaxies could sustain the observed rates of star formation over the 10 Gyr observed, but low-mass galaxies likely undergo episodic starbursts.