2025ApJ...983...94A

About two-thirds of galactic disks exhibit a central ellipsoidal stellar component called the bar, with or without a gaseous counterpart. However, there are a few dwarf galaxies with purely gaseous bars: NGC 3741, NGC 2915, and DDO 168. This is a puzzle, as gas is a collisional medium, and a gaseous bar is expected to be ripped off by shock waves. We study the formation of gaseous bars in these galaxies by constructing dynamical models constrained by stellar photometry and H I observations already available. We first analytically study the dynamical stability of the galactic disks against global m = 2 perturbations. Our results indicate that the stellar and gas disks are moderately unstable against these bar modes. Using N-body + hydrodynamical simulations employing RAMSES, we next find that a purely gaseous bar is formed in an oblate dark matter halo of vertical-to-planar axis ratio c/a = 0.6-0.8, with a relatively high-spin parameter Λ = 0.04-0.07, which survives for more than 10 dynamical times. Further, the low values of our calculated Mach numbers M = 2-6 of the gaseous medium comply with the survival of the gaseous bars, unaffected by shock waves. Interestingly, our simulations show the formation of a tiny stellar bar in each case. However, the temporal evolution of the change in angular momentum L_z of the different disk components indicates the exchange of L_z between the gas disk and the dark matter halo only; the L_z of the stellar disk remained unchanged, indicating a weak stellar bar.