Galaxies in extreme environments: Isolated galaxies versus compact groups.

摘要

This Dissertation comprises two distinct studies of galaxies in dramatically different environments: extreme isolation versus compact groups. We emphasize empirically how "nature" (i.e. internal, secular processes) plays the dominant role in defining the evolution of isolated galaxies and how "nurture" dictates the fate of galaxies in very crowded environments. Two chapters report on a detailed photometric study of a well-defined sample of N ∼100 isolated Sb-Sc spiral galaxies. Data source is Sloan Digital Sky Survey. Using i-band images we perform three kinds of measures: (a) bulge/disk/bar decomposition, (b) CAS parametrization (Concentration, Asymmetry, Clumpiness), and (c) Fourier decomposition/analysis of spiral arms and bar properties including dynamical measures of the torque. Having quantified a large set of properties we look for: (i) the interplay between different components of the same galaxy, (ii) trends along the morphological sequence Sb-Sbc-Sc, and (iii) statistical differences between our "isolated" sample and samples of galaxies of similar morphology constructed without regard for isolation. We find that the majority of isolated late-type disk galaxies host pseudobulges rather than classical bulges. The pseudobulges probably form through internal secular processes and bars may play an important role. A clear separation is noted between Sb and Sbc/Sc in various measures, i.e. the former are redder, brighter, have larger disks and bars, more luminous bulges, are more concentrated, more symmetric and dumpier than the latter. Isolated galaxies host larger bars than galaxies in samples defined without isolation constraints. Longer bars are not necessarily stronger, but show a higher contrast in Fourier analysis. Another chapter is a multiwavelength study of Seyfert's Sextet, the highest density galaxy aggregate in the local Universe. Four of its five galaxies are interpreted as remnant bulges of accreted spirals and are now embedded in a luminous halo. In spite of long standing theoretical predictions of a major merger of the galaxy members into a giant one, we find evidence that compact groups are in fact long-lived and form by sequential acquisition of galaxies, coalescing via slow dissolution.