Faint Members of Distant Galaxy Groups

Abstract

<p>In this thesis, we present an analysis of ten galaxy groups at z ~ 0.4 using spectroscopic data from the FORS2 instrument on the VLT. This study targets group member galaxies at magnitudes fainter than any previous study at this redshift. Our group sample comes from the Group Environment and Evolution Collaboration, which was originally based upon the second Canadian Network for Observational Cosmology redshift survey. Fifty-two new group members are identified, mostly within the apparent magnitude range of 21.25 < r < 23.25; this accounts for an approximate increase of 25 per cent in the membership of these groups. We combine these new data with previously obtained follow-up spectroscopy and an extensive set of multiwavelength data to compute composite group galaxy luminosity functions and red fractions. Results from the whole sample are compared with subsampIes defined by cuts in group velocity dispersion, radius, and absolute magnitude. Our results indicate that the group environment contains proportionately more bright galaxies than the field (but similar numbers of faint galaxies) and displays little evidence for strong evolution of the luminosity function upon a comparison with lower-redshift samples. In terms of colour, group galaxies are consistently redder than the field. However, there is a strong trend of decreasing red fraction toward fainter magnitudes, in which the group and field environment become statistically similar near the magnitude limit of our sample. The faintest galaxies in our sample are thus predominantly blue, and located in regions outside the virial radius as defined by <em>R₂₀₀</em>. Overall, our sample's colours follow a general trend of increasing group red fraction with decreasing redshift, as we show in a comparison with the literature.</p>