Gas Feedback on Stellar Bar Evolution

We analyze evolution of live disk-halo systems in the presence of various gasfractions, f_gas less than 8% in the disk. We addressed the issue of angularmomentum (J) transfer from the gas to the bar and its effect on the barevolution. 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 verticalbuckling instability in the gas-poor disks and by a steep heating of a stellarvelocity dispersion by the central mass concentration (CMC) in the gas-richdisks. The gas has a profound effect on the onset of the buckling -- largerf_gas brings it forth due to the more massive CMCs. The former process leads tothe well-known formation of the peanut-shaped bulges, while the latter resultsin the formation of progressively more elliptical bulges, for larger f_gas. Thesubsequent (secular) evolution of the bar differs -- the gas-poor modelsexhibit a growing bar while gas-rich models show a declining bar whose verticalswelling is driven by a secular resonance heating. The border line between thegas-poor and -rich models lies at f_gas ~ 3% in our models, but ismodel-dependent and will be affected by additional processes, like starformation and feedback from stellar evolution. The overall effect of the gas onthe evolution of the bar is not in a direct J transfer to the stars, but in theloss of J by the gas and its influx to the center that increases the CMC. Themore massive CMC damps the vertical buckling instability and depopulates orbitsresponsible for the appearance of peanut-shaped bulges. The action of resonantand non-resonant processes in gas-poor and gas-rich disks leads to a convergingevolution in the vertical extent of the bar and its stellar dispersionvelocities, 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