Kelkar, K.
(2017)
Structure, star formation history and environment of galaxies.
PhD thesis, University of Nottingham.
Abstract
This thesis probes the role of environment in galaxy evolution, focussing particularly on understanding the links between the truncation of star formation, the transformation of galaxy structure, and environment. This study utilises deep HST imaging, photometric and spectroscopic data for galaxies within ten high-z cluster fields, which form part of the ESO Distant Cluster Survey (EDisCS). I first compare the mass--size relations of cluster and field galaxies, to address the dependence of galaxy size on environment observed from z~2.5, and which seemingly disappear at lower redshifts. I find no significant difference in the size distributions of cluster and field galaxies of a given morphology, or with similar rest-frame B-V colours. I rule out average size differences larger than 10--20 % in both cases. Thus, I conclude that if the size difference at higher-z reported in the literature is real, the growth of field galaxies seem to have caught up with that of cluster galaxies by z~1. Any putative mechanism responsible for galaxy growth has to account for the existence of environmental differences at high redshift and their absence (or weakening) at lower redshifts.
I then move on to analyse the effects of the global cluster/field environment on the internal structure of galaxies and their colours. I introduce quantitative non-parametric measures like the residual flux fraction (RFF) and the asymmetry in galaxy residuals (A_res) which measure deviations from symmetric light distributions using HST images, to explore the internal structure of galaxies. I also obtain complementary information on the probable causes of structural disturbances, both internal and external in nature, by performing visual classifications of cluster and field galaxies. Combining these two approaches of measuring galaxy structure, it is found that the RFF is a good proxy for `roughness' in the surface brightness distribution, while A_ res is more sensitive to the causes of the structural disruption. Incorporating visual morphology and environment, it was found that the external causes of disturbances were most often associated with star formation in spiral galaxies. When adding information on the star formation activity of galaxies, I discover two complementary subpopulations of galaxies abundant in clusters: visually undisturbed passive spirals, and undisturbed star-forming lenticulars. In addition to being visually symmetric, these passive cluster spirals are also consistently smoother than their star-forming counterparts. These observations, therefore, strongly advocate gentle physical processes acting on the gas content to modify the star formation properties of galaxies accreted into clusters, without large-scale disturbances in their stellar structure.
Considering the variations of quantitative galaxy structure with the star formation history of galaxies, I find that the young, star-forming galaxies are consistently rougher and more asymmetric than the galaxies with older passive stellar populations. Further, the galaxies with different average stellar ages seem to have similar distributions of RFF and A_res over cluster/field environments, thereby emphasising that the star formation history of galaxies is strongly linked to their intrinsic structure alone.
Finally, complementing the global cluster/field environment, I explore the projected phase--space as a tool to investigate possible variations in galaxy structure and their stellar ages over the internal cluster environment. The analysis with the projected phase--space shows a decrease in the fraction of galaxies with younger stellar populations in the cluster core when separated by morphology, especially for spirals. This trend, however, is less pronounced in the observed distributions of RFF and A_res across the projected phase--space and the field.
All these observations, when put together, signify that the star formation in galaxies is shut down as they get accreted into clusters, while the internal structure of galaxies remains more or less unaffected. The actual morphological change in galaxies, therefore, will follow later after the star formation has already been truncated. Furthermore, the physical mechanisms driving these transformations would, therefore, be gas extinguishing ICM processes like ram-pressure stripping and starvation, which leave the galaxy structure undisturbed.
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