An optical and H i study of the dwarf Local Group galaxy VV124 = UGC4879

A gas-poor dwarf with a stellar disk?

¹ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: michele.bellazzini@oabo.inaf.it
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
³ Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Instituto de Astrofisica de Canarias, c/via Lactea s/n, La Laguna 38205, Spain
⁶ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
⁷ Institute for Theoretical Physics, University of Zürich, Winterthurer-strasse 190, 9057 Zürich, Switzerland
⁸ Institut für Astronomie, ETH Zürich-Hönggerberg, Wolfgang-Pauli-Strasse16, 8093 Zürich, Switzerland
⁹ INAF - Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ Dipartimento di Astronomia - Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 17 November 2010
Accepted: 9 December 2010

Abstract

We present a detailed study of the dwarf galaxy VV124 (UGC4879), recently recognized as a remarkably isolated member of the Local Group. We have obtained deep (r ≃ 26.5) wide-field (23′ × 23′) g,r photometry of individual stars with the LBC camera at the Large Binocular Telescope 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 μ_r ≃ 30 mag/arcsec²), 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 H i observations obtained with the Westerbork Synthesis Radio Telescope (WSRT), the first ever of this object. A total amount of  ≃ 10⁶   M_⊙ of H i 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 H i in the region superposed to the stellar main body of the galaxy is V_h =  −25 km s^-1. The velocity field shows substructures typical of galaxies of this size but no sign of rotation. The H i spectra indicates the presence of a two-phase interstellar medium, again typical of many dwarf galaxies.