NGC 5011C: An overlookd dwarf galaxy in the centaurus a group

A critical study of the properties of groups of galaxies can be done only once a complete census of group members is available. Despite extensive surveys, even nearby groups can lead to surprises. Indeed, we report the discovery of a previously unnoticed member of the Centaurus A Group, NGC 5011C. While the galaxy is a well-known stellar system listed with a NGC number, its true identity has remained hidden because of coordinate confusion and wrong redshifts in the literature. NGC 5011C attracted our attention since, at a putative distance of 45.3 Mpc, it would be a peculiar object with a very low surface brightness typical of a dwarf galaxy and, at the same time, a size typical of an early-type spiral or S0 galaxy. To confirm or reject this peculiarity, our immediate objective was to have the first reliable measurement of its recession velocity. The observations were carried out with EFOSC2 at the 3.6 m European Southern Observatory (ESO) telescope, and the spectra were obtained with the instrument in long-slit mode. The redshifts of both NGC 5011C and its neighbor NGC 5011B were computed by cross-correlating their spectra with that of a radial velocity standard star. We found that NGC 5011C indeed has alow redshift of v_⊙ = 647±96 km s^-1 and thus is a nearby dwarf galaxy rather than a member of the distant Centaurus cluster, as believed for the past 23 years. Rough distance estimates based on photometric parameters also favor this scenario. As a by-product of our study we update the redshift for NGC 5011B to v_⊙ = 3227±50 km s^-1. Applying population synthesis techniques, we find that NGC 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±0.1 Gyr and 1/5 solar. Finally, we estimate a stellar mass of NGC 5011C comparable to that of dwarf spheroidal galaxies in the Local Group.