The dependence of galaxy group star formation rates and metallicities on large-scale environment

We construct a sample of 75863 star-forming galaxies with robust metallicity and star formation rate (SFR) measurements from the Sloan Digital Sky Survey Data Release 7, from which we select a clean sample of compact group (CG) galaxies. The CGs are defined to be close configurations of at least four galaxies that are otherwise apparently isolated. Our selection results in a sample of 112 spectroscopically identified CG galaxies, which can be further divided into groups that are either embedded within a larger structure, such as a cluster or large group, or truly isolated systems. The CGs then serve as a probe into the influence of large-scale environment on a galaxy's evolution, while keeping the local density fixed at high values. We find that the SFRs of star-forming galaxies in CGs are significantly different between isolated and embedded systems. Galaxies in isolated systems show significantly enhanced SFR, relative to a control sample matched in mass and redshift, a trend not seen in the embedded systems. Galaxies in isolated systems exhibit a median SFR enhancement at a fixed stellar mass of +0.07 ± 0.03 dex. These dependences on large-scale environment are small in magnitude relative to the apparent influence of local-scale effects found in previous studies, but the significance of the difference in SFRs between our two samples constrains the effect of large-scale environment to be non-zero. We find no significant change in the gas-phase interstellar metallicity for either the isolated or embedded CG sample relative to their controls. However, simulated samples that include artificial offsets indicate that we are only sensitive to metallicity changes of log O/H> 0.13dex (at 99per cent confidence), which is considerably larger than the typical metallicity differences seen in previous environmental studies.