The stellar populations of early-type galaxies in groups and clusters

Abstract

This thesis investigates the relationship between the environment of an early-type galaxy and its stellar populations. Most importantly, we wish to determine whether the star formation histories of early-type galaxies depend on the density of the group or cluster to which they belong. This issue is of considerable consequence, as it provides a means to distinguish whether the early-type morphology of these systems is determined at their formation, or whether it is assumed at a later time due to the action of their changing surroundings. We have applied two methods to compare the stellar populations of these galaxies. Firstly, we have analysed the strengths of surface gravity sensitive spectral lines in order to compare the relative contributions of dwarf and giant stars to the blue light. Several samples of E and SO galaxies, drawn from a wide variety of environments, have been measured with this technique. Secondly, we have made a precise comparison of the broad-band colours of early-type galaxies in the Coma and Virgo clusters. These clusters have very different average densities, and distinctly disparate mixtures of galaxy morphological types. The findings of these studies appear to be contradictory, however. Although the surface gravity analysis indicates that early-type galaxies in lower density regions contain more substantial populations of relatively young stars, the broad-band colours of galaxies in the Virgo and Coma clusters are indistinguishable. A third section of the thesis develops the Press-Schechter theory for the evolution of gravitational structure in an expanding universe. Our work allows us to compare the past histories of the environments of galaxies as a function of the mass of the group or cluster to which they presently belong. In the final chapter, we discuss how the results from these three areas of research relate to each other, and suggest two ways in which the apparent paradox of our observational work may be resolved. Considering the implications of our theoretical work leads us to review the fundamental differences between the 'nature' and 'nurture' scenarios for the origin of galaxy morphology.- ii -