Avid: A Near-Major Post-Merger of Late-Type Dwarfs beneath a Regularly Rotating HI Disk (VCC 693)

In the periphery of galaxy clusters, the combination of moderately high galaxy number densities and modest velocity dispersions favors interactions and mergers that affect the galaxy evolution prior to cluster infall. Observational studies of this important phase of galaxy evolution, particularly in the dwarf galaxy regime, are still rare. We present a high-resolution case study of VCC 693 (stellar mass ∼ 2.8 × 108 M⊙), a merger remnant in the outskirts of the Virgo cluster, using observations from the project Atomic gas in Virgo Interacting Dwarf galaxies (AVID). We explored the consequences of the merger on the star formation and structural evolution of VCC 693, based on a joint analysis of high-resolution Karl G. Jansky Very Large Array and high-sensitivity Five-Hundred-meter Aperture Spherical radio Telescope H I emission line observations, optical broadband images, narrowband Hα images, and optical spectra. We also employed hydrodynamical simulations of dwarf-dwarf mergers to aid in interpreting the observations of VCC 693. Our analysis favors a near-major merger between two late-type dwarf galaxies with a stellar mass ratio of approximately 3:1–4:1, in which one of the progenitors is likely relatively gas poor (i.e., a damp merger). The optical appearance of VCC 693 is dominated by complex tidal structures spanning the whole system, whereas the H I gas has settled to a regular rotating disk with a normal surface density profile. Compared to dwarfs of similar mass, the star formation and gas-phase metallicity are moderately enhanced in the center. The global star formation rate, H I gas content, and H I-to-optical size ratio of VCC 693 are broadly consistent with those of typical late-type dwarfs of similar mass, although they fall at the lower side of the distributions. By decomposing the H I rotation curve into the baryonic and dark matter component, we found that the dark matter halo is characterized by an unusually high concentration or core density. This implies that the dark matter halo might have relaxed into a more centrally peaked distribution following the merger event. Together with two other recent studies of AVID post-merger systems, our findings reinforce the emerging view that even major mergers between dwarfs can produce remnants whose overall stellar structures, apart from tidal features, are indistinguishable from those of ordinary dwarfs, and that the diverse environmental effects experienced by galaxies in cluster outskirts can promote damp or mixed mergers, which constitute an integral part of galactic preprocessing.