Morphological Evolution of Disk Galaxies and Their Concentration, Asymmetry, and Clumpiness (CAS) Properties in Simulations across Toomre’s Q Parameter

Abstract

We investigate the morphological and structural evolution of disk galaxies in simulations for a wide range of Toomre’s Q parameters. In addition to the inspection of conventional bar modes, we compute the concentration, asymmetry, and clumpiness (CAS) parameters to enlarge the understanding of the galaxy evolution. These parameters are widely employed to analyze the light distribution of the observed galaxies, but the adaptation to numerical simulations is not much considered. While the bar formation takes place in a considerable range of Q around 1, barred galaxies originating from Q > 1 and Q < 1 disks yield CAS values that differ significantly. Disks starting with Q < 1 develop clumps due to local gravitational instabilities along with the bar, and these clumps play a central role in enhancing the CAS values. This process is absent in the Q > 1 counterparts in which the evolution is dominated by linearly unstable two-armed modes that lead to lower CAS values. Likewise, unbarred galaxies that are obtainable from disks with Q far below and far above 1 exhibit greatly different CAS magnitudes. It turns out that the CAS parameters can serve as indicators of the initial kinematical state and the evolution history of a disk of any morphology. In addition, we find an alternative mechanism of the formation of the lopsided barred galaxy when Q ≲ 1. Bars that evolve in the midst of the clumps can spontaneously become lopsided at the end.