Specific Star Formation Rates to Redshift 5 from the FORS Deep Field and the GOODS-S Field

We explore the build-up of stellar mass in galaxies over a wide redshiftrange 0.4 < z < 5.0 by studying the evolution of the specific star formationrate (SSFR), defined as the star formation rate per unit stellar mass, as afunction of stellar mass and age. Our work is based on a combined sample of ~9000 galaxies from the FORS Deep Field and the GOODS-S field, providing highstatistical accuracy and relative insensitivity against cosmic variance. As atlower redshifts, we find that lower-mass galaxies show higher SSFRs than highermass galaxies, although highly obscured galaxies remain undetected in oursample. Furthermore, the highest mass galaxies contain the oldest stellarpopulations at all redshifts, in principle agreement with the existence ofevolved, massive galaxies at 1 < z < 3. It is remarkable, however, that thistrend continues to very high redshifts of z ~ 4. We also show that withincreasing redshift the SSFR for massive galaxies increases by a factor of ~10, reaching the era of their formation at z ~ 2 and beyond. These findings canbe interpreted as evidence for an early epoch of star formation in the mostmassive galaxies, and ongoing star-formation activity in lower mass galaxies.Comment: Accepted for publication in ApJL; 4 pages, 2 color figures, uses emulateapj.cl