Surface [ITAL]BR[/ITAL] Photometry of Newly Discovered Dwarf Elliptical Galaxies in the Nearby Sculptor and Centaurus A Groups

Abstract

Prompted by the recent finding of a large number of gas-rich dwarf irregular galaxies in the nearby southern galaxy groups Sculptor (Scl) and Centaurus A (Cen A) we carried out a complementary survey to search for members of the gas-poor dwarf elliptical (dE) galaxy family. In total 18 dE candidates were identified on SRC Sky Survey films covering the group regions. For five dE's in the Scl group and five in the Cen A group, membership information is available from surface brightness fluctuations distance measurements; two dEs in each group also have independent redshift measurements. The remaining eight galaxies have been associated to the groups based on their morphology. We acquired deep B- and R-band CCD images for all galaxies and determined their photometric and structural parameters. According to the magnitude range covered, -14 < MB < -9 (not counting the bright dS0 NGC 59), these objects are local dwarf spheroidal (dSph) analogues. The surface brightness profiles of most dwarfs are strongly nonexponential but are perfectly explained by the three-parametric Sersic law. The best-fitting Sersic parameters are compared with corresponding data for Virgo dE's and dSph's. The present dEs follow the relations between the Sersic parameters and absolute B-band magnitude for Virgo dE's and local dwarfs, thus confirming the membership in the Scl and Cen A groups, respectively. Most of our dwarfs exhibit color gradients in (B-R)0, in the sense of a reddening outward. We also show and discuss the sky distributions and B-band luminosity functions of the Scl and Cen A groups. The completeness magnitude for both populations (from the present survey) is estimated to be M ≈ -13. We find that the composite luminosity function of four nearby galaxy groups is well fitted by a Schechter function down to M = -14 with a faint end slope α = -1.29(±0.10). In the appendices we present best-fitting Sersic profile parameters for the dSph members of the Local Group and introduce two new dwarf irregulars found in the Cen A group.