The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf–Dwarf Merging

Abstract

It has long been speculated that many starburst or compact dwarf galaxies are resulted from dwarfdwarf galaxy merging, but unequivocal evidence for this possibility has rarely been reported in the literature. We present the first study of deep optical broadband images of a gas-dominated blue compact dwarf galaxy (BCD) VCC848 (M 2×10 8 M ) which hosts extended stellar shells and thus is confirmed to be a dwarf-dwarf merger. VCC848 is located in the outskirts of the Virgo Cluster. By analyzing the stellar light distribution, we found that VCC848 is the result of a merging between two dwarf galaxies with a primary-to-secondary mass ratio 5 for the stellar components and 2 for the presumed dark matter halos. The secondary progenitor galaxy has been almost entirely disrupted. The age-mass distribution of photometrically selected star cluster candidates in VCC848 implies that the cluster formation rate (CFR, ∝ star formation rate) was enhanced by a factor of ∼ 7 − 10 during the past ∼ 1 Gyr. The merging-induced enhancement of CFR peaked near the galactic center a few hundred Myr ago and has started declining in the last few tens of Myr. The current star formation activities, as traced by the youngest clusters, mainly occur at large galactocentric distances ( 1 kpc). The fact that VCC848 is still (atomic) gas-dominated after the period of most violent collision suggests that gas-rich dwarf galaxy merging can result in BCD-like remnants with extended atomic gas distribution surrounding a blue compact center, in general agreement with previous numerical simulations.

Accretion of galaxy groups into galaxy clusters

“…However, it is encouraging that within the limited resolution of these simulations a few cases arise that may compare well with these peculiar objects found in Virgo (e.g. Paudel et al 2017;Zhang et al 2020).…”

“…Such mergers will typically involve two dwarf galaxies with similar masses and should contain gas to fuel the central starburst that gives rise to the dense inner blue core. On the other hand, gas-free similar mass dwarf-dwarf mergers would explain better the presence of low surface brightness shells as found in a couple of dwarfs in deep studies of the outskirts of Virgo (Paudel et al 2017;Zhang et al 2020).…”

“…This is perhaps important to help explain the observational evidence for young mergers in nearby galaxy clusters like Virgo (e.g. Zhang et al 2020). We follow the assembly of our 10 clusters in Illustris and identify possible merger events, their mass ratios, times and location with respect to the cluster centre.…”

“…Moss 2006). For instance, the presence of shell dwarf galaxies in the outskirts of Virgo (Paudel et al 2017;Zhang et al 2020) is a vivid reminder that mergers can occur in galaxy clusters.…”

“…Mergers have been postulated as a viable mechanism to transform morphology, in particular in galaxy types that are commonly found in galaxy clusters, such as lenticular (S0) galaxies (Bekki 1998;D'Onofrio, Marziani & Buson 2015;Eliche-Moral et al 2018) or blue compact dwarfs (BCDs; Bekki 2008;Zhang et al 2020). Groups of galaxies infalling together will have shallower gravitational potentials than the cluster, allowing for lower velocity interactions to occur and, eventually, mergers (see e.g.…”