Gas Feedback on Stellar Bar Evolution

We have analyzed the evolution of live disk-halo systems in the presence of various fractions of gas, fgas < 8% of the disk mass. Specifically, we have addressed the issue of angular momentum (J) transfer from the gas to the stellar 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 former process leads to the well-known formation of the boxy/peanut-shaped bulges, while the latter results in the formation of progressively more elliptical bulges. The subsequent evolution of the bar differs -- 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 fgas ~ 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 dynamical and secular 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 CMC damps the vertical buckling instability in the bar and depopulates orbits responsible for the appearance of boxy/peanut-shaped bulges. The combined action of resonant and non-resonant processes in gas-poor and 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.