Origin of Nonaxisymmetric Features of Virgo Cluster Early-type Dwarf Galaxies. II. Tidal Effects on Disk Features and Stability

Abstract

Abstract 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 VCC 856. We consider eight cluster–galaxy models in which disk dwarf galaxies with differing pericenter distance and spin orientation experience the tidal gravitational force of a Virgo-like Navarro–Frenk–White halo, as well as an additional eight galaxy–galaxy models in which two dwarf galaxies undergo tidal interactions with different strength. We find that the cluster tidal effect is moderate owing 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 disk–halo system are 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 VCC 856 are best matched with the arms in a marginally unstable galaxy produced by a distant tidal encounter with its neighbor ∼0.85 Gyr ago.