Near-ultraviolet signatures of environment-driven galaxy quenching in Sloan Digital Sky Survey groups

We have investigated the effect of group environment on residual star formation in galaxies, using Galaxy Evolution Explorer near-ultraviolet (NUV) galaxy photometry with the Sloan Digital Sky Survey group catalogue of Yang et al. We compared the (NUV - r) colours of grouped and non-grouped galaxies, and find a significant increase in the fraction of red sequence galaxies with blue (NUV - r) colours outside of groups. When comparing galaxies in mass-matched samples of satellite (non-central), and non-grouped galaxies, we found a > 4σ difference in the distribution of (NUV - r) colours, and an (NUV - r) blue fraction > 3σ higher outside groups. A comparison of satellite and non-grouped samples has found the NUV fraction is a factor of ~2 lower for satellite galaxies between 10^10.5 and 10^10.7M_⊙, showing that higher mass galaxies are more likely to have residual star formation when not influenced by a group potential. There was a higher (NUV - r) blue fraction of galaxies with lower Sérsic indices (n < 3) outside of groups, not seen in the satellite sample. We have used stellar population models of Bruzual & Charlot with multiple burst, or exponentially declining star formation histories to find that many of the (NUV - r) blue non-grouped galaxies can be explained by a slow (~2 Gyr) decay of star formation, compared to the satellite galaxies. We suggest that taken together, the difference in (NUV - r) colours between samples can be explained by a population of secularly evolving, non-grouped galaxies, where star formation declines slowly. This slow channel is less prevalent in group environments where more rapid quenching can occur.