The Dependence of the Evolution of Early‐Type Galaxies on Their Environment

Abstract

This thesis describes the analysis of a data set of 121 elliptical and lenticular (S0) galaxies, located in different environments and at distances corresponding to look-back times between 2 and 5 Gyrs. Such a sample allows to study the environmental dependence of the internal properties of these early-type galaxies to draw conclusions on the formation epoch and evolutionary history of their sub-classes of elliptical and S0 galaxies.As a discrimination factor for the environment, the X-ray luminosity flux is utilised, which is proportional to the square of the mass of a galaxy. The highest galaxy density is found in rich galaxy clusters, (low-mass) poor galaxy clusters exhibit a factor of 10 lower number density whereas the lowest density is defined by individual, isolated field galaxies. Theoretical predictions based on a hierarchical structure growth suggest for various environments different evolutionary tracks which yield to a diversity of galaxy ages.All objects analysed in this work were selected based upon deep ground-based multi-band imaging data. In addition, for the central regions of the galaxy clusters and the two extended sky areas which comprise the field galaxies, the FORS Deep Field and the William Herschel Deep Field, the high resolution images of the Hubble Space Telescope (HST) provide a detailed morphological classification and structural parameter analysis. Multi-Object-Spectroscopy was conducted for all galaxies at the 8.2-m Very Large Telescope (VLT, Chile) and the Calar Alto 3.5-m telescope (Spain). Using high signal-to-noise spectroscopy the internal velocity dispersions (i.e., the random motion of the individual stars) of the galaxies are measured, which are an indicator for the total galaxy masses. Based on different scaling relations, in particular the Faber-Jackson relation between velocity dispersion and luminosity, the absolute brightnesses of the distant galaxies are compared to galaxies in the local universe. While high-mass elliptical and S0 galaxies show only a mild evolution, the distant low-mass galaxies are more luminous by up to a factor of 2 with respect to their local counterparts as seen today.The observed mass-dependence of the luminosity evolution is found in all environments. By contrast, numerical simulations based on the hierarchical scenario expect only for the isolated field galaxy population a significant increase of the luminosity towards higher redshifts that is independent on the galaxy type. Elliptical and S0 galaxies are not a homogeneous galaxy group but follow different evolutionary tracks. Lenticular galaxies indicate in all environments a stronger evolution and more recent formation epochs (1<zform≤2), whereas elliptical galaxies evolve passively on longer timescales yielding to older stellar population ages (zform≈3). Possible explanations for the deviations could be different stellar populations or more complex, extended star formation histories in the case of S0 galaxies. The discrepancies between the predicted and observed luminosity evolution are most likely due to a lack of sufficiently realistic modelling of the involved processes of the stellar content in the simulations.