The Star-formation-rate-Density Relation at 0.6 < z < 0.9 and the Role of Star-forming Galaxies

Abstract

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 σ _z /(1 + z) ~ 1% for galaxies with z _AB < 23.3 mag. We utilized a stellar mass-limited sample of 977 galaxies above M > 1.8 × 10¹⁰ M _sun (log M/M _sun >10.25) to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24 μm imaging, (2) spectral energy distribution (SED) fitting, and (3) [O II]λ3727 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 [O II]-based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS-based SFRs, the decline is a factor of ~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 ~ 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 ~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 ~5-6 for the SED- and [O II]-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 A_V , 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.