Evolution of Galaxies Probed by their Structure

Abstract

Stellar structures of galaxies provide us valuable information on how galaxies evolved. With an aim to understand the evolution of galaxies, we investigate stellar structures in various types of galaxies. We have performed two-dimensional (2D) multicomponent decomposition of 144 local moderately face-on barred spiral galaxies using 3.6 μm images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single exponential function in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T ≥ 0.1, the break radius to bar radius (rbr/Rbar) varies between 1 and 3, but as a function of B/T, the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad Resonance (OLR) of the bar, and thus moves outwards as the bar grows. For galaxies with small bulges, B/T < 0.1, rbr/Rbar spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds. We have measured the radial light profiles and 2D shapes of bars quantitatively. The surface brightness profile of bar is correlated with B/T. Bars in massive and bulge-dominated galaxies (B/T > 0.2) show flat profiles (Sersic nbar ∼0.3) while bars in less massive and disk-dominated galaxies (B/T ∼ 0) primarily show exponential, disk-like profiles (Sersic nbar ∼ 0.85) with a wider spread in the radial profile than in the bulge-dominated galaxies. The 2D shapes of the bars, however, are rectangular/boxy, independent of the bulge or disk properties. Our results are consistent with the observed cosmological evolution of barred galaxies in that more massive galaxies formed their bars first and had enough time to arrange resonances and to trap stars into their bar orbits and these lead to flatten bar profiles from initial exponential profile. The narrow spread in the bar radial profiles and bar shapes in the more massive disks suggests that these bars formed earlier (z > 1), while the disk-like profiles and a larger spread in the radial profile in lower mass systems implies a later and more gradual evolution, consistent with studies of disk assembly and evolution. We propose that a combination of the light profile combined with vertical stellar velocity dispersion may be used as a viable chronometer for the dynamical age of a bar. Tidal debris around galaxies can yield important clues on their evolution. We have identified tidal debris in 11 early type galaxies (ETGs, T ≤ 0) from a sample of 65 ETGs drawn from the S4G. A variety of techniques, including 2D decomposition of galactic structures, was used to quantify the residual tidal features. We investigate the locations of galaxies with tidal debris in the Fundamental Plane and Kormendy relation. We find that galaxies with tidal debris lie within the scatter of ETGs without tidal features. Assuming that the tidal debris is indicative of recent merger, this suggests that these galaxies have undergone minor merging events so that the overall structural properties of the galaxies are not significantly altered, or they have undergone a major merging events but already have experienced sufficient relaxation and phase-mixing so that their structural properties become similar to those of the non-interacting ETGs. Our results also imply that structural properties of the central parts of ETGs (red nuggets) have not been significantly modified through merging event, which is consistent with the two-phase scenario of galaxy formation.