Specific Star Formation Rates to Redshift 1.5

We present a study to determine how star formation contributes to galaxygrowth since z=1.5 over five decades in galaxy stellar mass. We investigate thespecific star formation rate (SSFR; star formation rate [SFR] per unit galaxystellar mass) as a function of galaxy stellar mass and redshift. A sample of175 K-band selected galaxies from the MUnich Near-Infrared Cluster Surveyspectroscopic dataset provide intermediate to high mass galaxies (mostly M* >10^10 Msun) to z=1. The FORS Deep Field provides 168 low mass galaxies (mostlyM* < 10^10 Msun) to z=1.5. We use a Sloan Digital Sky Survey galaxy sample totest the compatibility of our results with data drawn from a larger volume. Wefind that at all redshifts, the SSFR decreases with increasing galaxy stellarmass suggesting that star formation contributes more to the growth of low massgalaxies than to the growth of high mass galaxies, and that high mass galaxiesformed the bulk of their stellar content before z=1. At each epoch we find aridge in SSFR versus stellar mass that is parallel to lines of constant SFR andevolves independently of galaxy stellar mass to a particular turnover mass.Galaxies above this turnover mass show a sharp decrease in the SFR compared tothe average at each epoch and the turnover mass increases with redshift. TheSFR along the SSFR ridge decreases by roughly a factor of 10, from 10 Msun/yrat z=1.5 to 1 Msun/yr at z=0. High mass galaxies could sustain the observedrates of star formation over the 10 Gyr observed, but low mass galaxies likelyundergo episodic starbursts.Comment: 4 pages, 2 figures, matches ApJL accepted versio