In-depth Analysis of Bar Formation Mechanisms of Disk Galaxies in Halos of Different Concentrations

Abstract We use N -body simulations to investigate the distinct bar formation processes in disks residing in halos of various concentrations. In a highly concentrated halo, the bar development is limited by the dominant multiarm modes as a result of the swing amplification in the early stage. After the multiarm modes decay, the bar growth proceeds mechanically owing to the particle trapping in continuation of that bar seed. In this scheme, the corotation resonance of the bar modes does not come into play at all, justified by a low amount of disk–halo angular momentum transfer and a modestly decreasing bar pattern speed. On the other hand, although reducing the halo concentration suggests the reduction of the preferred swing-amplified modes to be bisymmetric, the bar formation in a low-concentration halo does not involve the swing amplification at all. Rather, the fast-growing linearly unstable bar modes of a single uniform frequency are the sole governing factor, attributed to a mild shearing. The bar modes have triggered the corotation resonance since the beginning, and such resonance is maintained until the end, which leads to a high amount of angular momentum transfer and a fast slowdown. For the intermediate halo concentration, the kinematical analyses of multiple nonaxisymmetric modes suggest that the linear modes, the swing amplification, and the particle trapping are all present in the evolution chronology. To specify bars formed in the different halo concentrations, full analyses of the isophotal shape, the radial Fourier amplitude, and the resonance diagram can be of use.