Probing the star formation history of early-type galaxies in clusters

Abstract

In this thesis, we present a new photometric catalogue of the local Coma galaxy cluster in the Johnson U- and V- bands. We cover an area of 3360arcmin(^2) of sky, to a depth of V = 20mag in a 13 arcsec diameter aperture, and produce magnitudes for ~ 1400 extended objects in metric apertures from 8.8 to 26arcsec diameters. The mean internal RMS scatter in the photometry is 0.014mag in V, and 0.026mag in U, for Via(_13) < 17mag.We use this photometric catalogue to place limits on the levels of scatter in the colour- magnitude relation (CMR) in the Coma cluster. We subdivide the galaxy population by morphology, luminosity and position on the sky, and analyse the CMR in each of them. The lowest levels of scatter are found in the elliptical galaxies, and the late type galaxies have the highest numbers of galaxies blue-wards of the CMR. We finds signs of decreased scatter and systematically bluer galaxy colours with increasing projected radius from the center of the cluster, and attribute it to a mean galactic age gradient. We find that the typical mass of galaxies within clusters can increase by a factor of two through dissipationless merging without destroying the CMR. We compare the spectral line indices of galaxies in the Coma cluster with their deviation from the mean colour-magnitude relation (CMR). We find that the CMR in Coma is driven primarily by a luminosity-metallicity correlation, however we cannot rule out a contribution from age effects. Colour deviations blue-ward of the mean relation are strongly correlated with the Hydrogen Baimer line series absorption, indicating the presence of a young stellar population in these blue galaxies. We use a wavelet code to suggest an association between X-ray cluster substructure and 'E+A' galaxy activity in high redshift clusters.