Jaffe, Yara Lorena
(2012)
The formation and evolution of galaxies as a function of environment.
PhD thesis, University of Nottingham.
Abstract
This thesis investigates the role of environment on galaxy formation and evolution, giving particular focus to the transformation of star forming spirals into passive S0s. The data utilised for this study comes from photometric and spectroscopic observations of galaxies at 0 < z < 1 in different environments from the ESO Distant Cluster Survey. We first study the formation history of (172) cluster ellipticals (Es) and S0s, the oldest types of galaxies in the local universe. We examine their colour-magnitude relation (CMR), and find a very small intrinsic colour scatter. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with z or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that while it does not depend on the cluster velocity dispersion, it increases weakly with cluster redshift. This suggests that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars ≃2–4Gyr prior to observation. In other words, the galaxies that already have early-type morphologies also have reasonably-old stellar populations. This is partly a manifestation of the "progenitor bias", but also a consequence of the fact that the vast majority of the E/S0s in clusters (in particular the massive ones) were already red by the time they achieved their morphology. Moreover, E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter E/S0s finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the E/S0s cluster galaxies must have had their star formation truncated over an extended period ∆t ≿1 Gyr.
We then move our focus to the evolution of star-forming galaxies. We investigate the effect of the environment on the transformation of star-forming spirals into passive S0s by studying the properties of the gas and the stars in a sample of 422 emission-line galaxies in different environments. We identify galaxies with kinematical disturbances (in their gas disks), and find that they are more frequent in clusters than in the field. The fraction of kinematically-disturbed galaxies increases with cluster velocity dispersion and decreases with distance from the cluster centre, but remains constant with projected galaxy density. We also studied morphological disturbances in the stellar light, finding that the fraction of morphologically disturbed galaxies is independent of environment. Moreover, there is little correlation between the presence of kinematically-disturbed gas and morphological distortions. For the kinematically-undisturbed galaxies, we find that the cluster and field B-band Tully-Fisher relations are remarkably similar. Additionally, we find that the kinematically-disturbed galaxies show a suppressed specific star formation rate. There is also evidence indicating that the gas disks in cluster galaxies have been truncated, and therefore their star formation is more concentrated than in low-density environments. If spirals are the progenitors of cluster S0s, our findings imply that the physical mechanism transforming cluster galaxies efficiently disturbs the star forming gas and reduces their specific star formation rate. This star-forming gas is either removed more efficiently from the outskirts of the galaxies or it is driven towards the centre (or both). In any case, this makes any remaining star formation more centrally concentrated, helping to build the bulges of S0s. All this evidence, together with the fact that the transformation mechanism does not seem to induce strong morphological disturbances on the galaxies, suggests that the physical processes involved are related to the intracluster medium, with galaxy-galaxy interactions playing only a limited role in clusters. Interestingly, in analogy with the "blue" early-type galaxies found in the CMR study in clusters, we have also found several emission-line E/S0 galaxies with extended rotating star-forming gas disks.
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