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

Abstract

On the periphery of galaxy clusters, moderately high galaxy densities and velocity dispersions favour interactions and mergers that influence galaxy evolution prior to cluster infall. Observational studies of this phase in dwarfs remain rare. We present a high-resolution study of the merger remnant VCC 693 in the outskirts of Virgo cluster, using observations from the Atomic gas in Virgo Interacting Dwarf galaxies (AVID) project. We explore the origin of VCC 693 and the consequences of the merger on its star formation and structure through a joint analysis of VLA and FAST HI emission line observations, together with complementary optical imaging and spectroscopy. We employ hydrodynamical simulations to help interpret the observations. Our analysis favours a near-major merger between two dwarfs with a stellar mass ratio of 3:1-4:1, with one likely gas-poor progenitor (i.e., a damp merger). The optical appearance of VCC 693 is dominated by complex tidal structures throughout the system, whereas the HI gas has settled to a regular rotating disk. Compared with similar-mass dwarfs, the central star formation and gas-phase metallicity are moderately enhanced. The global star formation rate, HI gas content, and HI-to-optical size ratio of VCC 693 are broadly consistent with those of typical dwarfs of similar mass, albeit somewhat lower. Decomposition of the HI rotation curve into baryonic and dark matter indicates a high halo concentration, suggesting post-merger relaxation into a more centrally peaked configuration. Together with two recent studies of AVID post-merger systems, these results support the view that even major dwarf mergers can produce remnants with overall stellar structures indistinguishable from ordinary dwarfs, and that the environmental effects in cluster outskirts can promote damp or mixed mergers, constituting an integral part of galactic pre-processing.

Figures (20)

• Figure 1: Left panel: the NGVS three-color image of VCC 693, produced by combining the u-, g-, and i-bands. Right panel: the unsharp-masked image of VCC 693.
• Figure 2: Left panel: The optical image from DESI of the environment of VCC 693. Two galaxies are located to the northwest (VCC 613) and southeast (VCC 785) of VCC 693, whereas a dwarf galaxy is located to the northeast (VCC 764). The dashed lines show their virial radii estimated as 67 times the r-band half-light radius. The blue arrows indicate the directions and angular distances to the centre of the Virgo cluster (M87) and the nearest substructure (W cloud), relative to VCC 693. Right panel: Projected phase-space diagram of redshift-confirmed Virgo galaxies. The escape velocity profiles of Virgo as a function of the angular distance from M87 are indicated by the two black dashed lines. VCC 693 is shown in red, overlaid on the Virgo cluster members displayed in black.
• Figure 3: Left panel: NGVS g-band image after masking background galaxies. Middle panel: Diagram showing u-band surface brightness versus $u-g$ colors. The analysis is limited to pixels brighter than $\mu_{g}$ = 25.4 mag arcsec$^{-2}$, and the pixels are divided into four tranches according to their $u-g$ colors. The color boundaries between blue, green, yellow, and red tranches are 0.35, 0.65, and 0.95. Right panel: Spatial distribution of the $u-g$ colors, where the colors correspond to those of the tranches in the middle panel. The ellipse in the left and right panels marks the location of the stellar bar.
• Figure 4: Isophotal analysis of the NGVS images of VCC 693. Left: The radial profiles of the ellipticity (top) and position angle (PA; bottom). Right: The radial profiles of the i-band surface brightness (top) and $g-i$ color (bottom). The red line shows the Sérsic profile best fit to the surface brightness profile, with the fitted values of effective radius $R_{\rm e,i}$ and effective surface brightness $\mu_{\rm e,i}$ are shown in the panel. The blue shadow regions in the two left panels mark the region used to derive the disk $\varepsilon$ and PA.
• Figure 5: The H$\alpha$ flux density maps from VESTIGE (left) and MUSE (right) observations. The MUSE observations from the archives cover only about a third of the southern region of VCC 693, as shown in the white rectangle. The black circles mark the locations and sizes of SDSS and BOSS spectra fibers. The ellipse in the left panel approximately marks the location of the stellar bar. We identified the star-forming regions with H$\alpha$ emission and measured their metallicity and H$\alpha$ velocity, as indicated in the top right and bottom right panels, respectively. The MUSE star-forming regions are indicated with ellipses of different colors. Spectra of the bottom-left two star-forming regions (in green or blue) and of the rightmost three regions (in yellow) are summed up respectively to increase the S/N.