The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf-Dwarf Merging: HI Gas, Star Formation and Numerical Simulations

TL;DR

This study combines observations and simulations to explore how a dwarf-dwarf merger in VCC 848 influences its gas distribution and star formation, revealing a complex interaction history and its effects on galaxy evolution.

Contribution

It provides the first detailed analysis linking a gas-rich dwarf-dwarf merger to star formation and gas dynamics using combined observational data and numerical simulations.

Findings

Less than 30% of HI gas is in the central star-forming region.

A tidal arm contains HI but no star formation.

Simulations suggest a merger between a gas-dominated and a star-dominated galaxy over 1 Gyr ago.

Abstract

A clear link between a dwarf-dwarf merger event and enhanced star formation (SF) in the recent past was recently identified in the gas-dominated merger remnant VCC 848, offering by far the clearest view of a gas-rich late-stage dwarf-dwarf merger. We present a joint analysis of JVLA HI emission-line mapping, optical imaging and numerical simulations of VCC 848, in order to examine the impact of the merger on the stellar and gaseous distributions. VCC 848 has less than 30% of its HI gas concentrated within the central high-surface-brightness star-forming region, while the remaining HI is entrained in outlying tidal features. Particularly, a well-defined tidal arm reaches N(HI) comparable to the galaxy center but lacks SF. The molecular gas mass inferred from the current SF rate (SFR) dominates over the atomic gas mass in the central ~ 1.5 kpc. VCC 848 is consistent with being a main-sequence star-forming galaxy for its current stellar mass and SFR. The HII region luminosity distribution largely agrees with that of normal dwarf irregulars with similar luminosities, except that the brightest HII region is extraordinarily luminous. Our N-body/hydrodynamical simulations imply that VCC 848 is a merger between a gas-dominated primary progenitor and a gas-bearing star-dominated secondary. The progenitors had their first passage on a near-radial non-coplanar orbit more than 1 Gyr ago. The merger did not build up a core as compact as typical compact dwarfs with centralized starburst, which may be partly ascribed to the star-dominated nature of the secondary, and in a general sense, a negative stellar feedback following intense starbursts triggered at early stages of the merger.