An optical and HI study of the dwarf Local Group galaxy VV124=UGC4879. A gas-poor dwarf with a stellar disk?

TL;DR

This study provides a comprehensive optical and HI analysis of the isolated dwarf galaxy VV124, revealing an extended stellar disk, a low gas content, and complex gas kinematics, contributing new insights into its structure and composition.

Contribution

It is the first detailed optical and HI study of VV124, uncovering its extended stellar disk, low HI mass, and complex gas dynamics, which challenge previous assumptions about gas-poor dwarf galaxies.

Findings

VV124 has an extended stellar disk with a radius of 1.9 kpc.

The galaxy contains a low HI mass of 10^6 M_sun, with M_HI/L_V=0.11.

HI gas shows no rotation and exhibits a two-phase medium.

Abstract

We present a detailed study of the dwarf galaxy VV124, recently recognized as a isolated member of the Local Group. We have obtained deep (r=26.5) wide-field g,r photometry of individual stars with the LBT under sub-arcsec seeing conditions. The Color-Magnitude Diagram suggests that the stellar content of the galaxy is dominated by an old, metal-poor population, with a significant metallicity spread. A very clean detection of the RGB tip allows us to derive an accurate distance of D=1.3 +/- 0.1 Mpc. Combining surface photometry with star counts, we are able to trace the surface brightness profile of VV124 out to ~ 5' = 1.9 kpc radius (where mu_r=30 mag/arcsec^2), showing that it is much more extended than previously believed. Moreover, the surface density map reveals the presence of two symmetric flattened wings emanating from the central elongated spheroid and aligned with its major axis, resembling a stellar disk seen nearly edge-on. We also present HI observations obtained with the WSRT, the first ever of this object. A total amount of 10^6 M_sun of HI gas is detected in VV124. Compared to the total luminosity, this gives a value of M_HI/L_V=0.11, which is particularly low for isolated Local Group dwarfs. The spatial distribution of the gas does not correlate with the observed stellar wings. The systemic velocity of the HI in the region superposed to the stellar main body of the galaxy is V_h=-25 km/s. The velocity field shows substructures typical of galaxies of this size but no sign of rotation. The HI spectra indicates the presence of a two-phase interstellar medium, again typical of many dwarf galaxies.