After 54 years of bar instability studies: a fresh surprise

TL;DR

This paper revisits the long-standing problem of bar instability in rotationally supported disks by contrasting two modeling regimes: rigid halos, where nonlinear interference from faster-growing spiral modes can erroneously suppress bar formation, and live halos, where the bar instability reasserts itself. Using high-resolution 2D simulations with multiple sectoral harmonics and varied initial conditions, the authors show that suppression of bars is a numerical artifact tied to rigid halos and limited mode access, not a universal stabilizing mechanism. In 3D live-halo runs, the classic ELN stability criterion largely reappears, with bar formation modulated by swing-amplifier physics and halo–disk angular-momentum exchange; some live halos can still resist bar formation, indicating the outcome is sensitive to halo responsiveness and spiral activity. The work clarifies the conditions under which mode interference can alter bar growth and emphasizes the crucial role of halo dynamics in real galaxies, while leaving open the possibility that exceptionally vigorous spiral activity in certain halos could provide a novel bar-stabilizing pathway. $V_c^{\max}$ and $X$-parameter analyses are used to connect the simulations to established swing-amplifier theory and Lindblad-resonance dynamics.

Abstract

The well-known bar instability of rotationally-supported disk galaxy models has been studied extensively since its first discovery over half a century ago. We were therefore very surprised to find cases of disks embedded in rigid halos, which on the basis of widely-cited criteria should be unstable, that appeared to be robustly stable. Here we show that the unstable bar mode in such simulations was being suppressed by changes to the disk caused by other instabilities having higher angular symmetry that were the first to saturate. Although this may seem like a promising solution to the long-standing puzzle presented by the apparent stability of real disk galaxies, we also show that instability is restored in the same models when the rigid halo is replaced by a live population of particles, where the usual stability conditions apply. Our study has been confined to a narrow range of models, and we cannot therefore exclude the possibility that mode interference may be able to prevent bar formation in other models having live halos.

Figures (8)

• Figure 1: The evolution of a noisy start realization of the baseline model from SC23 with force terms $1 \leq m \leq 8$ all active. The color scale indicates the logarithm of the disk surface density. Notice that no strong bar forms, though a short, weak bar is visible from time to time.
• Figure 2: The radial variation of $Q$ at intervals of 50 dynamical times in the simulation illustrated in Fig. \ref{['fig.run5772']}. The lines are not labeled because increases in $Q$ are mostly monotonic.
• Figure 3: The evolution of one comparison simulation of model A having $N=60$K particles. The initial disk extends to $R=5R_d$ and times are given in dynamical times.
• Figure 4: Top: the mean amplitudes of the bar-like $A(2,0,t)$ (eq. \ref{['eq.logspi']}) in 10 noisy start realizations of model A, for 4 different values in $N$ with unrestricted forces. Middle: the same as for the top panel, but for $m=3$. Bottom: the evolution of the bar amplitude in identical noisy start simulations when non-axisymmetric forces are restricted to $m=2$. Results from the individual simulations were shifted in time in the bottom panel only so that $A=0.06$ at $t=100$ for each before averaging.
• Figure 5: Three example rotation curves (black curves) of disk + compressed halo. The disk contribution, which is the same in all three is shown in red, and the compressed halo in green, with $V_0=0.6$ in the left panel, $V_0=0.7$ in the middle panel, and $V_0= 0.8$ in the right panel. In all three panels the contribution of the halo before compression, for which $r_c = R_d/2$ is shown by the dotted curve.