The Evolution of Distant Spiral Galaxies in the FORS Deep Field

Abstract

This thesis describes the analysis of a data set of 130 spiral galaxies at distances corresponding to look-back times between 1 and 8 Gyrs. Such a sample allows to test the predictions of the hierarchical scenario, according to which small structures have formed first in the early cosmic stages, followed by the successive build-up of larger structures via merging and accretion events. This model is a paradigm of modern astrophysics.All objects analysed in this work are taken from the FORS Deep Field, a sky area that has been observed via deep multi-band imaging with the Very Large Telescope (VLT, Chile). Using spatially resolved VLT spectroscopy, rotation curves (i.e., the rotation velocity as a function of galactocentric radius) are extracted. 77 galaxies show a flat maximum of their rotation velocities at large radii, indicating that - similar to galaxies in the local universe - their mass distributions are dominated by dark matter in the outer parts. Based on the Tully-Fisher relation between maximum rotation velocity and luminosity, the absolute brightnesses of the distant galaxies are compared to galaxies in the local universe. While fast rotating, i.e., high-mass galaxies do not show a significant evolution, the distant low-mass objects are more luminous by up to factor of 6 with respect to their local counterparts. To the contrary, numerical simulations based on the hierarchical scenario either find an increasing luminosity towards higher redshifts for high-mass galaxies, or no correlation at all.Several tests are applied to the observed mass-dependence of the luminosity evolution. It is found that the result cannot be induced by an incompleteness effect, tidal interactions or false assumptions on the intrinsic absorption due to dust. Moreover, it offers an explanation for the discrepancies between previous studies which comprised too small samples to robustly test a mass-dependence. Since slightly smaller disk sizes are found towards higher redshifts - in compliance with the hierarchical scenario -, the deviations between the predicted and observed luminosity evolution are most likely due to a not sufficiently realistic modelling of the stellar (i.e. baryonic) component in the simulations.