We run self-consistent simulations of Milky Way-sized, isolated disk galaxies to study formation and evolution of a stellar bar as well as a nuclear ring in the presence of gas. We consider two sets of models with cold or warm disks that differ in the radial velocity dispersions, and vary the gas fraction f gas by fixing the total disk mass. A bar forms earlier and more strongly in the cold disks with larger f gas , while gas progressively delays the bar formation in the warm disks . The bar formation enhances a central mass concentration which in turn makes the bar decay temporarily, after which it regrows in size and strength, eventually becoming stronger in models with smaller f gas . Although all bars rotate fast in the beginning, they rapidly turn to slow rotators. In our models, only the gas-free, warm disk undergoes rapid buckling instability, while other disks thicken more gradually via vertical heating. The gas driven inward by the bar potential readily forms a star-forming nuclear ring. The ring is very small when it first forms and grows in size over time. The ring star formation rate is episodic and bursty due to feedback, and well correlated with the mass inflow rate to the ring. Some expanding shells produced by star formation feedback are sheared out in the bar regions and collide with dust lanes to appear as filamentary interbar spurs. The bars and nuclear rings formed in our simulations have properties similar to those in the Milky Way.

A fraction of early-type dwarf galaxies in the Virgo cluster have a disk component and even possess disk features such as bar, lens, and spiral arms. In this study, we construct 15 galaxy models that resemble VCC856, which is considered to be an infalling progenitor of disk dwarf galaxies, within observational error ranges, and use N-body simulations to study their long-term dynamical evolution in isolation as well as the formation of bar in them. We find that dwarf disk galaxies readily form bars unless they have an excessively concentrated halo or a hot disk. This suggests that infalling dwarf disk galaxies are intrinsically unstable to bar formation, even without any external perturbation, accounting for a population of barred dwarf galaxies in the outskirts of the Virgo cluster. The bars form earlier and stronger in galaxies with a lower fraction of counter-streaming motions, lower halo concentration, lower velocity anisotropy, and thinner disk. Similarly to normal disk galaxies, dwarf disk galaxies also undergo recurrent buckling instabilities. The first buckling instability tends to shorten the bar and to thicken the disk, and drives a dynamical transition in the bar pattern speed as well as mass inflow rate. In nine models, the bars regrow after the mild first buckling instability due to the efficient transfer of disk angular momentum to the halo, and are subject to recurrent buckling instabilities to turn into X-shaped bulges.

A fraction of dwarf galaxies in the Virgo cluster contain disk features like bars and spiral arms. Using N -body simulations, we investigate the effects of tidal forces on the formation of such disk features in disk dwarf galaxies resembling VCC856. We consider 8 Cluster-Galaxy models in which disk dwarf galaxies with differing pericenter distance and spin orientation experience the tidal gravitational force of a Virgo-like NFW halo, and additional 8 Galaxy-Galaxy models in which two dwarf galaxies undergo tidal interactions with different strength. We find that the cluster tidal effect is moderate due to the small galaxy size, making the bars form earlier by ∼ 1-1.5 Gyr compared to the cases in isolation. While the galactic halos significantly lose their mass within the virial radius due to the cluster tidal force, the mass of the stellar disks is nearly unchanged, suggesting that the inner regions of a diskhalo system is secured from the tidal force. The tidal forcing from either the cluster potential or a companion galaxy triggers the formation of two-armed spirals at early time before a bar develops. The tidally-driven arms decay and wind with time, suggesting that they are kinematic density waves. In terms of the strength and pitch angle, the faint arms in VCC856 are best matched with the arms in a marginally unstable galaxy produced by a distant tidal encounter with its neighbor ∼ 0.85 Gyr ago.