VV124 (UGC4879): A new transitional dwarf galaxy in the periphery of the Local Group

TL;DR

VV124 is an isolated dwarf galaxy in the Local Group, with detailed resolved-star photometry revealing its structure, stellar populations, and a closer distance estimate of 1.1 Mpc, indicating a transitional dIrr/dSph type.

Contribution

First resolved-star photometry and spectroscopic analysis of VV124, providing new insights into its distance, structure, and stellar populations, and classifying it as a transitional dwarf galaxy.

Findings

Distance to VV124 is 1.1 Mpc, closer than previously thought.

Contains both young blue stars and evolved red populations.

Properties suggest a transitional dIrr/dSph galaxy.

Abstract

We present the first resolved-star photometry of VV124 (UGC4879) and find that this is the most isolated dwarf galaxy in the periphery of the Local Group. Based on imaging and spectroscopic follow up observations with the 6m BTA telescope, we resolve VV124 into 1560 stars down to the limiting magnitude levels of V~25.6 and I~23.9. The young blue stellar populations and emission gas are found near the core, but noticeably displaced from the center of the galaxy as traced by dominant evolved red stars. The mean radial velocity derived from the spectra of two Blue Supergiant stars, an HII region and unresolved continuum sources is -80+/-10 km/s. The evolved ``red tangle'' stellar populations, which contains the red giant branch (RGB), are identified at large galactocentric radii. We use the I-band luminosity function to determine the distance based on the Tip of RGB method, 1.1+/-0.1 Mpc. This is ~10 times closer than the values usually assumed in the literature, and we provide revised distance dependent parameters. From the mean (V-I) color of the RGB, we estimate the mean metallicity as [Fe/H]~-1.37 dex. Despite of its isolated location, the properties of VV124 are clearly not those of a galaxy in formation, but rather similar to a transitional dIrr/dSph type.