In-depth analysis of bar formation mechanisms of disk galaxies in halos of different concentrations

TL;DR

The paper investigates how the central concentration of a dark matter halo influences bar formation in disk galaxies using live-halo N-body simulations. By varying the halo scale radius $r_h$ and analyzing Fourier amplitudes, spectrograms, and resonance diagrams, it identifies three distinct bar-formation pathways: a dynamics-based route in highly concentrated halos, a linear-mode-driven route in weakly concentrated halos, and a mixed regime at intermediate concentrations. In the high-concentration case, initial swing-amplified multi-arm modes give way to bar growth via particle trapping with negligible disk–halo angular-momentum transfer and little corotation resonance; in the low-concentration case, fast-growing linear $m=2$ modes trigger corotation from the start and drive significant angular-momentum transfer and rapid bar slowdown; the intermediate regime shows a combination of these processes and a transitional behavior tracked by a new diagnostic, the shearing-to-rigid ratio $\Xi$. The results offer a framework for interpreting observed bar morphologies and kinematics across galaxies and motivate using $\Xi$ and resonance analyses as diagnostic tools.

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 multi-arm modes as a result of the swing amplification in the early stage. Only after the multi-arm 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 bi-symmetric, the bar formation in a lowly concentrated halo does not involve the swing amplification at all. Rather, the fast-growing linearly unstable bar modes of a single uniform frequency is solely the governing factor, attributed to a mild shearing. The bar modes trigger 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 non-axisymmetric modes suggests 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.

Figures (8)

• Figure 1: Time evolutions of $A_{2}, A_{3}$ and $A_{4}$ for different indicated cases. The horizontal dashed line designates the value of $0.2$, serving as the threshold of $A_{2}$ for the barred state.
• Figure 2: Disk surface densities in color map for R35 (top row), R50 (middle row), and R75 (bottom row) at different times, in units of $9.3\times 10^{7} \ M_{\odot}/$kpc$^{2}$. The isodensity contours in the last two snapshots represent, from the outside to the inside, the levels of $0.2\Sigma_{0}, 0.4\Sigma_{0}$ and $0.8\Sigma_{0}$, where $\Sigma_{0}\equiv M_{d}/2\pi R_{0}^{2}$ corresponds to the central surface density of the initial disk.
• Figure 3: Radial profiles of the bar amplitude $\tilde{A}_{2}(R)$ for R35 (left panel), R50 (middle panel), and R75 (right panel) at the indicated times. The horizontal line indicates the value of $0.2$.
• Figure 4: Fourier spectrograms of the $m=2$ and $3$ modes for R35, R50, and R75 (see labels), calculated in the time windows indicated on each panel. The Fourier amplitude is represented by the isolines of the levels of $0.7$ and $0.6$ times the maximum amplitude in the disk interior, from darker to lighter color for each mode. The solid and dashed lines correspond to the numerically computed disk angular frequency $\Omega$ and $\Omega\pm\kappa/m$, respectively, determined within the indicated time windows.
• Figure 5: Top row: Probability distribution functions of the angular frequencies of the trapped particles $P(\omega_{i})$ for R35 (left panel), R50 (middle panel), and R75 (right panel) at different times (see text for the description of the trapped particles). Bottom row: standard deviation $s$ (plotted with the left axis in magenta color) and skewness $\alpha$ (plotted with the right axis in blue color) for the cases on the upper panels at the indicated times.