THE STAR-FORMATION-RATE-DENSITY RELATION AT 0.6 < z < 0.9 AND THE ROLE OF STAR-FORMING GALAXIES

We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6 < z < 0.9. We used a low-dispersion prism in IMACS on the 6.5 m Baade (Magellan I) telescope to obtain spectra and measured redshifts to a precision of sigma(z)/(1 + z) similar to 1% for galaxies with z(AB) < 23.3 mag. We utilized a stellar mass-limited sample of 977 galaxies above M > 1.8 x 10(10) M-circle dot (logM/M-circle dot > 10.25) to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24 mu m imaging, (2) spectral energy distribution (SED) fitting, and (3) [OII]lambda 3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED- and [OII]-based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS-based SFRs, the decline is a factor of similar to 4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z similar to 0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star-forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U-V and V-J colors to distinguish quiescent galaxies from SFGs (including both unattenuated blue galaxies and reddened ones), we find that the fraction of quiescent galaxies increases from similar to 32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U-V and V-J colors, to decline with increasing density by factors of similar to 5-6 for the SED-and [OII]-based SFRs. The MIPS-based SSFRs for SFGs decline with a shallower slope. The declining SFRs of SFGs with density are paralleled by a decline in the median AV, providing indirect evidence that the cold gas content that fuels future SF is diminished in higher density environments. The order of magnitude decline in the SSFR-density relation at 0.6 < z < 0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies.