DISCOVERY of A CLOSE PAIR of FAINT DWARF GALAXIES in the HALO of CENTAURUS A

Abstract

As part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), we report the discovery of a pair of faint dwarf galaxies (CenA-MM-Dw1 and CenA-MM-Dw2) at a projected distance of ∼90 kpc from the nearby elliptical galaxy NGC 5128 (CenA). We measure a tip of the red giant branch distance to each dwarf, finding D = 3.63 ± 0.41 Mpc for CenA-MM-Dw1 and D = 3.60 ± 0.41 Mpc for CenA-MM-Dw2, both of which are consistent with the distance to NGC 5128. A qualitative analysis of the color-magnitude diagrams indicates stellar populations consisting of an old, metal-poor red giant branch (≳12 Gyr, [Fe/H] ∼ -1.7 to -1.9). In addition, CenA-MM-Dw1 seems to host an intermediate-age population as indicated by its candidate asymptotic giant branch stars. The derived luminosities (MV = -10.9 ± 0.3 for CenA-MM-Dw1 and -8.4 ± 0.6 for CenA-MM-Dw2) and half-light radii (rh = 1.4 ± 0.04 kpc for CenA-MM-Dw1 and 0.36 ± 0.08 kpc for CenA-MM-Dw2) are consistent with those of Local Group dwarfs. CenA-MM-Dw1's low central surface brightness (μV, 0 = 27.3 ± 0.1 mag arcsec-2) places it among the faintest and most extended M31 satellites. Most intriguingly, CenA-MM-Dw1 and CenA-MM-Dw2 have a projected separation of only 3 arcmin (∼3 kpc): we are possibly observing the first, faint satellite of a satellite in an external group of galaxies.