The Role of Galaxy Environment in Quenching Galaxy Star Formation

Abstract

How galaxies transition between blue, star-forming, spirals and red elliptical galaxies is a key question in modern extragalactic astrophysics. Galaxies will transition between the two broad populations under competing influences including internal structure, gravitational interactions, black hole feedback, and group and cluster environment. These different processes will cause galaxies to transform in different ways, creating a diverse population of transition galaxies. This thesis aims to determine how environment creates transition galaxies, and how star formation is suppressed as galaxies transition between the blue and red populations. We show that in massive clusters, galaxies with residual star formation reside preferentially in cluster outskirts using the LARCS survey combined with Galex observations. We find ~ 5% of optically red galaxies in dense cluster cores with (NUV - R < 5.4), which indicated residual star formation. This fraction of red galaxies with residual star formation increases to 15% at large cluster radii and low surface density. We interpret this as a quenching of star formation as galaxies enter cluster environments. We observe (NUV - R) bright fractions of over 50% for morphologically spiral red sequence galaxies, and no preference for merging or edge on systems. This suggests most residual star formation in red galaxies is caused via a quenching of star formation in disc galaxies, and not a recent burst of star formation. We compare the fraction of galaxies with (NUV - r < 5) in different environments, using a group catalogue from the Sloan Digital Sky Survey. We find a > 3 sigma suppression of residual star formation in group galaxies compared with non-grouped galaxies. The (NUV - r) blue fraction is a factor of ~ 2 higher in the non-group sample for galaxies with low Sersic indices and stellar masses of ~1010.5 Msun. Many non-grouped galaxies with residual star formation have colours that match stellar population model tracks with > 1 Gyr timescale star formation decay. By contrast, grouped galaxies with residual star formation do not need a contribution from long decay truncations, implying a suppression of slow quenching galaxies in group environments compared with non-grouped galaxies. We measured the environments of rapid quenching galaxies in the GAMA survey, using rapidly quenching galaxies selected with E+A spectroscopic criteria and UV-optical photometric criteria. Although slow quenching is relatively rare in clusters, both the spectroscopic and photometric samples show no significant preference for rapidly quenching galaxies to reside in groups relative to a mass and colour matched comparison sample. The (NUV - r) colours of E+A galaxies however, are an average of 0.5 mag (> 3 sigma) redder than comparison galaxies, indicating that E+A galaxies are indeed more rapidly quenched than other transitioning galaxies. Our results demonstrate that environment does play a role in suppressing galaxy star formation, which is seen via the radial dependence of residual star formation in clusters and the suppression of slow quenching in groups. However, the variety of environments of rapidly quenching galaxies (selected with spectroscopic E+A and photometric UV-optical criteria), highlights that mechanisms correlated with environment are not the only means of suppressing star formation. Other mechanisms must drive the rapid quenching of star formation, and cause a fast transition to the red sequence.