Gas Feedback on Stellar Bar Evolution

Abstract

We analyze evolution of live disk-halo systems in the presence of various gas fractions, f_gas less than 8% in the disk. We addressed the issue of angular momentum (J) transfer from the gas to the bar and its effect on the bar evolution. We find that the weakening of the bar, reported in the literature, is not related to the J-exchange with the gas, but is caused by the vertical buckling instability in the gas-poor disks and by a steep heating of a stellar velocity dispersion by the central mass concentration (CMC) in the gas-rich disks. The gas has a profound effect on the onset of the buckling -- larger f_gas brings it forth due to the more massive CMCs. The former process leads to the well-known formation of the peanut-shaped bulges, while the latter results in the formation of progressively more elliptical bulges, for larger f_gas. The subsequent (secular) evolution of the bar differs -- the gas-poor models exhibit a growing bar while gas-rich models show a declining bar whose vertical swelling is driven by a secular resonance heating. The border line between the gas-poor and -rich models lies at f_gas ~ 3% in our models, but is model-dependent and will be affected by additional processes, like star formation and feedback from stellar evolution. The overall effect of the gas on the evolution of the bar is not in a direct J transfer to the stars, but in the loss of J by the gas and its influx to the center that increases the CMC. The more massive CMC damps the vertical buckling instability and depopulates orbits responsible for the appearance of peanut-shaped bulges. The action of resonant and non-resonant processes in gas-poor and gas-rich disks leads to a converging evolution in the vertical extent of the bar and its stellar dispersion velocities, and to a diverging evolution in the bulge properties.Comment: 12 pages, 12 figures, accepted for publication by the Astrophysical Journal. Minor corrections following the referee repor

Evolution of spiral galaxies in modified gravity

“…Contrary to what was found by Berentzen et al (2007), the introduction of gas does not damp the buckling instability, in the models considered here (both in MOND and DM). In MOND, without gas, the mass concentration in the Sc-model is lower in the centre, and the position of the vertical instability occurs at a larger radius, leading to a less conspicuous box-peanut shape in the edge-on view.…”

“…On the average, it appears 2−3 Gyr after the beginning of the simulation in the DM model against 4−5 Gyr in the MOND model. Bars are weakened during the buckling (Berentzen et al 2007), but this is not the only way. The MOND-Sc simulation with gas does not have a strong peanut, but the bar is weakened because the instability is too violent during a short period: the bar strength increases to 0.4 in less than 1 Gyr.…”

MUSE tells the story of NGC 4371: The dawning of secular evolution

“…For example, Cheung et al (2015) found "no significant differences in the stellar populations of the bulges or the gradients of barred versus unbarred quiescent disk galaxies". However, modern simulations at least indicate that bars, once formed, are difficult to dissolve (in the absence of major mergers), unless the disc is extremely gas-rich Bournaud et al 2005;Berentzen et al 2007;Kraljic et al 2012).…”

Dynamics of Disks and Warps

“…Alternatively, the bar could have grown in size and slowed trhough disk evolution (Sellwood 1981;Berentzen et al 2007) or halo friction (Athanassoula 2002, see §9.8), or have been triggered by large density fluctuations in the disk (Sellwood 1989a), by tidal interactions (Noguchi 1987;Berentzen et al 2004;Curir et al 2006), halo substructure (Romano-Díaz et al 2008b) or a non-axisymmetric halo (Dubinski & Chakrabarty 2009). Any of these considerations, or that described in the next paragraph, could plausibly reconcile the existence of bars in these galaxies with Toomre's stabilizing mechanism.…”