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We run self-consistent simulations of Milky Way-sized, isolated disk galaxiesto study formation and evolution of a stellar bar as well as a nuclear ring inthe presence of gas. We consider two sets of models with cold or warm disksthat differ in the radial velocity dispersions, and vary the gas fraction$f_{\rm gas}$ by fixing the total disk mass. A bar forms earlier and morestrongly in the cold disks with larger $f_{\rm gas}$, while gas progressivelydelays the bar formation in the warm disks . The bar formation enhances acentral mass concentration which in turn makes the bar decay temporarily, afterwhich it regrows in size and strength, eventually becoming stronger in modelswith smaller $f_{\rm gas}$. Although all bars rotate fast in the beginning,they rapidly turn to slow rotators. In our models, only the gas-free, warm diskundergoes rapid buckling instability, while other disks thicken more graduallyvia vertical heating. The gas driven inward by the bar potential readily formsa star-forming nuclear ring. The ring is very small when it first forms andgrows in size over time. The ring star formation rate is episodic and burstydue to feedback, and well correlated with the mass inflow rate to the ring.Some expanding shells produced by star formation feedback are sheared out inthe bar regions and collide with dust lanes to appear as filamentary interbarspurs. The bars and nuclear rings formed in our simulations have propertiessimilar to those in the Milky Way.

Effects of Gas on Formation and Evolution of Stellar Bars and Nuclear Rings in Disk Galaxies