Ngc 5011 C : an Overlooked 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 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 having a very low surface brightness typical of a dwarf galaxy, and at the same time having the size 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 3.6m 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 has indeed a low redshift of v⊙ = 647± 96 km sec −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 byproduct of our study we update the redshift for NGC 5011B at v⊙ = 3227 ± 50 km sec . 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. Subject headings: galaxies: clusters: individual: CenA group – galaxies: dwarf – galaxies: individual: NGC 5011B, NGC 5011C