Stellar populations of local cluster early-type galaxies

Abstract

This thesis investigates local cluster early-type galaxies, combining new spectroscopy with multi-wavelength imaging. We probe the stellar content of elliptical and lenticular galaxies, exploring the interconnection between age, chemical composition and dynamical substructure. We analyse the ultraviolet-infrared colour-magnitude relation, using new GALEX imaging of quiescent red-sequence galaxies in local clusters, and confirm that the intrinsic scatter is an order of magnitude larger than for the analogous optical relation. We compare the UV-IR colours to spectroscopic stellar populations parameters, and find a strong correlation with metallicity (albeit still with a large scatter), and only a marginal trend with age. We argue that the UV upturn is not significant in this sample, and demonstrate that the intrinsic scatter could be attributed to simple frosting by either a young or low metallicity subpopulation. We present a comprehensive study of the internal gradients in age, metallicity and a-element abundance for 25 cluster early-type galaxies, using data from the VLT VIMOS integral field unit. We find negative metallicity gradients, with a large intrinsic scatter for galaxies with σ ≥ 130 km s-(^1) which we speculate could be the lower limit for a formation history dominated by major mergers. Stellar population gradients are primarily related to the central metallicity: early-type galaxies with super-solar centres have steep negative metallicity gradients and positive age gradients; those with solar metallicity centres have negligible [Z/H] gradients and negative age gradients. B-R colour gradients predicted from the spectroscopic age and metallicity generally agree well with those measured directly from photometry. There is a strong observed anti-correlation between the gradients in age and metallicity. While a part of this trend can be attributed to correlated measurement errors, we demonstrate that there is an underlying intrinsic relation. We present new CMOS long-slit observations of seven edge-on lenticular galaxies in the Coma cluster, probing to several disc scale lengths in each. We strongly confirm that these S0s are significantly offset in luminosity from the spiral Tully-Fisher relation, and show that the size of this offset is correlated with projected local density. However, there appears to be no difference in the mean offset between S0 samples from various global environments (i.e. group and cluster). We derive the radial trends in the stellar populations, and find generally regular profiles consistent with those observed in the VIMOS sample. Significant deviations from the general radial trends coincide with structural and kinematic boundaries. We find that the age of the disc component is correlated with the offset from the Tully-Fisher relation, and agrees well with simple models of abrupt star formation truncation in a spiral disc. We show that SO discs tend to be older and more metal poor than the central regions, supporting the theory of bulge growth alongside disc quenching, during S0 formation