Origin of Non-Axisymmetric Features of dE Galaxies in the Virgo Cluster

Abstract

A fraction of early-type dwarf galaxies in the Virgo cluster has a disk component and even possesses disk features such as bar, lens, and spiral arms. Using N-body simulations, we propose formation scenarios of these non-axisymmetric features in the disky dwarf galaxies. By adopting VCC 856 as our progenitor, a bulgeless dwarf disk galaxy with embedded faint spiral arms, we construct 15 initial conditions with slight dynamical variations based on observational error range. After 10 Gyrs of evolution in isolation, our standard model slowly forms a bar at ~3 Gyr and then undergoes buckling instability that temporarily weakens the bar, although the bar continues to grow afterward. 13 of our isolated models are also unstable to bar formation and undergo buckling instability. This suggests that the disky dwarf galaxies are intrinsically unstable to form bars, accounting for a population of barred dwarf galaxies in the outskirts of the Virgo cluster. We also find that both the concentration of dark matter halo and the degree of random motions affect the vigor of buckling instability. The regrowth of bars is observed in our models, and its condition is closely related to the angular momentum transfer and the amount of mass accreted into the center after the first buckling instability. To understand the origin of the faint grand-design spiral arms, we additionally construct 6 sets of tidal models by differing pericenter distances. We reveal that its formation mechanism is rather more complicated: the faint spiral arms consistent with the observations could develop on marginally unstable disk by relatively weak tidal force. Additionally, we construct 8 sets of major and minor merger models to examine transformation of bars into lens-like structures. Our merger models indicate merging direction from the major axis of bars and volume density of infalling perturbers are vital to partially dissolve bars and form lens-like structure. We mainly discuss our results in light of morphological and dynamical evolution of disky dwarf galaxies.