Environmental effects at early cosmic times: galaxy evolution in the first massive clusters
Final Report Abstract
This project has investigated galaxy population properties in very massive galaxy clusters at z~1.5. The studied cluster sample is made of the highest redshift clusters identified in the South Pole Telescope Sunyaev-Zel’dovich survey (SPT-SZ). With masses M200~5 1014 M⊙ these are among the rarest, most massive clusters known at these redshifts. The SZ cluster selection is independent of cluster galaxies and is considered as a proxy to cluster mass selection, making this sample well suited for selection-unbiased galaxy evolution studies. Based on a specific investigation in the context of this study we indeed conclude that the cluster sample studied here can be considered as representative of the general galaxy cluster population in the probed cluster mass and redshift range. This project was based on a dedicated, homogeneous follow-up imaging program with the Hubble and Spitzer Space Telescopes, securing observations in four optical/NIR passbands in the cluster central regions. Based on these observations we have measured stellar masses and rest-frame optical structural parameters of cluster galaxies, and broadly classified them as galaxies with active or significantly suppressed star formation. In the context of the study of the effect of cluster environments accelerating the evolution of galaxies towards bulge-dominated systems with low star formation, we have thus investigated the predominance of galaxies with suppressed star formation (quiescent galaxies) and bulge-dominated morphologies in the central regions of these clusters corresponding to a look-back time of 9-10 billion years. We find that, albeit with some cluster to cluster variation, the fraction of quiescent sources among massive galaxies in the central cluster regions is typically ≳80%, to be compared with a quiescent fraction of ~50-60% for galaxies of same stellar mass and redshift in average-density “field” regions. This implies that, already 9-10 billion years ago, the average star formation histories of galaxies in cluster environments were different from counterparts in the field, with galaxy evolution processes in clusters resulting in an efficient suppression of star formation in cluster galaxies. In agreement with some previous results in clusters at similar redshift, these findings do not lend support to widespread claims in the literature of the past decade that cluster environments at high redshift are no longer effective at suppressing star formation even in their core regions. We note though that our findings refer to the central volume of the most massive clusters at z~1.5: the dependence of the efficiency of cluster environments at suppressing star formation on cluster mass is still an important matter of investigation. We have then investigated the structural properties of massive cluster galaxies in the same central regions in terms of their broad morphology as quantified by their Sersic index, and of galaxy size. We find a clear correlation between broad stellar population and structural properties, with the bulk of quiescent and starforming galaxies showing, respectively, bulge- and disk-dominated morphologies. Because of the high quiescent galaxy fraction in the clusters with respect to the field, this correlation results in a morphologydensity relation qualitatively similar to what observed at lower redshifts, with a higher predominance of bulge-dominated galaxies in cluster with respect to field environments. On the other hand, we find that the relative predominance of bulge- and disk-dominated galaxies when considering separately the quiescent and star-forming populations is fully consistent between cluster and field. This suggests that although star formation is preferentially suppressed in clusters, the mechanisms producing the correlation between star formation suppression and structural evolution towards a bulge dominated morphology are still related in cluster regions as in the field, with a difference in the relative timescales that is not significant in our measurements. We have further investigated the relation between galaxy size and stellar mass in the cluster central regions and its potential difference with its field counterpart, in particular for quiescent, bulge-dominated galaxies, which has been the subject of significant debate over the past decade. We find substantial consistence of the mass-size relation of homogeneous populations in clusters and in the field, meaning that average galaxy sizes at fixed stellar mass are consistent for galaxies with the same broad stellar population properties and morphology. On the other hand, the overall different quiescent galaxy and bulge-dominated fractions in cluster vs. field environments may result in a different distribution of galaxies on the mass-size plane if using more general galaxy samples not explicitly pre-selecting homogeneous sub-populations in a consistent way. An investigation of stellar mass assembly across the virial volume of the observed clusters is still ongoing. This project was critical to motivate or enable a range of follow-up studies, among which we are currently particularly focusing on: 1) the investigation of the environmental quenching efficiency across the virial region, based on recently completed observations specifically acquired as a follow-up to this work; and 2) a dedicated, consistent comparison of results from this work with theoretical expectations from cosmological simulations, to improve the interpretation of observations of galaxy evolution in cluster environments within the cosmological context.
