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

Abstract

We present the first photometry of individual stars within VV124 (UGC4879) and find that this is the most isolated dwarf galaxy in the periphery of the Local Group. This work is based on imaging and spectroscopic follow-up observations with the 6-m BTA telescope; we resolve VV124 into 1560 stars down to the limiting magnitude levels of V≃ 25.6 and I≃ 23.9. Young blue stellar populations and ionized gas are found near the core, but notably displaced from the centre of the galaxy as traced by dominant evolved red stars. The mean heliocentric radial velocity derived from the spectra of two blue supergiant stars and unresolved continuum sources is −70 ± 15 km s−1. The evolved ‘red tangle’ stellar populations, which contain 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) colour of the RGB, we estimate the mean metallicity as [Fe/H]≃−1.37 dex. Despite 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.