The effect of the environment on the evolution of galaxy colours

Abstract

In chapter 1 of this thesis, I give a brief overview of the birth of modern cosmology and the growth of the current paradigm. I then go on to describe the different types of galaxies, the star formation rates within them and give some notes about galaxy photometry. In chapter 2 I give a brief overview of the current generation of high redshift galaxy surveys and describe the data, the analysis of which forms the original work in this thesis. This work is summarised below. In chapter 3 I describe the methods I used to carry out the work. I present my results in chapter 4 and my conclusions in chapter 5.I present an analysis of the environmental dependence of the u'-r' colour distribution of 433 galaxies in a volume limited sample. The median redshift is 0.8. The sample was created from the Deep Extragalactic Evolutionary Probe 1 (DEEP) Groth Strip survey with magnitude and redshift limits given by M(_B) < -19.0 and z < 1.1 respectively. The environment was quantified using the spatial density of galaxies, measured in number Mpc(^-3). Comparison of the results to those obtained by other authors from the Sloan Digital Sky Survey (SDSS) around z = 0.06 permits an analysis of the evolution of this colour distribution. The colour distributions from both surveys are well fitted by the sum of two Gaussian distributions, delineating two distinct galaxy populations. These are the early-types (red) and the late-types (blues). My first result is that the fraction of galaxies that are in the red population increases with environmental density. The proportion of galaxies that are red increases from 0.2 to 0.4 over 4 orders of magnitude of environmental density. That trend is much stronger at lower redshift. Further, the difference between the fractions of red galaxies in the two redshift regimes is greatest in the cluster environments. This difference is evidence of the Butcher-Oemler Effect. The mean colours of the two populations change very little as density increases. However, perhaps a very faint trend of the blue galaxies getting redder with increasing density is detected. Further analysis indicates that the change in galaxies' colours with cosmic time is almost entirely due to processes intrinsic to the galaxies themselves. Their environments make an almost negligible contribution to the colour change.