THE EVOLUTION OF THE SPECIFIC STAR FORMATION RATE OF MASSIVE GALAXIES TO z similar to 1.8 IN THE EXTENDED CHANDRA DEEP FIELD SOUTH

We study the evolution of the star formation rate (SFR) of mid-infrared selected galaxies in the extended Chandra Deep Field South. We use a combination of UBVRIz' JHK GaBoDS and MUSYC data, deep Infrared Array Camera observations from SIMPLE, and deep MIPS data from FIDEL. This unique multiwavelength data set allows us to investigate the SFR history of massive galaxies out to redshift z similar to 1.8. We determine the SFRs using both the rest-frame ultraviolet luminosity from young, hot stars and the total infrared IR luminosity of obscured star formation obtained from the MIPS 24 mu m flux. We find that at all redshifts the galaxies with higher masses have substantially lower specific SFRs (SSFRs) than lower-mass galaxies. The average SSFRs increase with redshift, and the rate of incline is similar for all galaxies (roughly (1 + z)(n), n = 5.0 +/- 0.4). It does not seem to be a strong function of galaxy mass. Using a subsample of galaxies with masses M-* > 10(11) M-circle dot, we measured the fraction of galaxies whose star formation is quenched. We consider a galaxy to be in quiescent mode when its SSFR does not exceed 1/(3 x t(H)), where t(H) is the Hubble time. The fraction of quiescent galaxies defined as such decreases with redshift out to z similar to 1.8. We find that, at that redshift, 19% +/- 9% of the M-* > 10(11) M-circle dot sources would be considered quiescent according to our criterion.