NGC 5011C: An Overlooked Dwarf Galaxy in the Centaurus A Group

(abridged) We report the discovery of a previously unnoticed member of theCentaurus A Group, NGC 5011C. While the galaxy is a well known stellar systemlisted with a NGC number its true identity remained hidden because ofcoordinate confusion and wrong redshifts in the literature. NGC 5011C attractedour attention since, at a putative distance of 45.3 Mpc, it would be a peculiarobject having a very low surface brightness typical of a dwarf galaxy, and atthe same time having the size of an early-type spiral or S0 galaxy. To confirmor reject this peculiarity, our immediate objective was to have the firstreliable measurement of its recession velocity. The observations were carriedout with EFOSC2 at the 3.6m ESO telescope. We found that NGC 5011C has indeed alow redshift of v_sun=647+/-96 km/sec and thus is a nearby dwarf galaxy ratherthan a member of the distant Centaurus cluster as believed for the past 23years. Rough distance estimates based on photometric parameters also favor thisscenario. As a byproduct of our study we update the redshift for NGC 5011B atv_sun=3227+/-50 km/sec. Applying population synthesis techniques, we find thatNGC 5011B has a luminosity-weighted age of 4+/-1 Gyr and a solar metallicity,and that the luminosity-weighted age and metallicity of NGC 5011C are 0.9+/-0.1Gyr and 1/5 solar. Finally we estimate a stellar mass of NGC 5011C comparableto that of dwarf spheroidal galaxies in the Local Group.Comment: 13 pages, 6 figures, accepted by the Astronomical Journa