Structure and stellar content of dwarf galaxies. III: B and R photometry of dwarf galaxies in the M101 group and the nearby field

Abstract

We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf galaxies in and around the M101 group. Eleven are members and suspected members of the group and ten are field galaxies in the projected vicinity of the group. We have derived total magnitudes, effective radii, effective surface brightnesses, as well as galaxy diameters at various isophotal levels in both colours. Best-fitting exponential parameters and colour gradients are also given for these galaxies. Some of the galaxies show a pronounced luminosity excess above the best-fitting exponential at large radii, or surface brightnesses fainter than approx 26 mag/sq_arcsec in R. This feature, while non-significant for a single case and technically difficult to interpret, might be an indication of the existence of an extended old stellar halo in dwarf irregulars. The photometric parameters of the galaxies presented here will be combined with previously published data for nearby dwarf galaxies and statistically analysed in a forthcoming paper.