VLT surface photometry and isophotal analysis of early-type dwarf galaxies in the Virgo cluster

We have carried out surface photometry and an isophotal analysis for a sample of 25 early-type dwarf (dE and dS0) galaxies in the Virgo cluster based on CCD images taken at the VLT with FORS1 and FORS2. For each galaxy we present B and R-band surface brightness profiles, as well as the radial colour (B - R) profile. We give total apparent BR magnitudes, effective radii, effective surface brightnesses and total colour indices. The light profiles have been fitted with Sérsic models and the corresponding parameters are compared to the ones for other classes of objects. In general, dEs and dSOs bridge the gap in parameter space between the giant ellipticals and the low-luminosity dwarf spheroidals in the Local Group, in accordance with previous findings. However, the observed profiles of the brightest cluster dwarfs show significant deviations from a simple Sérsic model, indicating that there is more inner structure than just a nucleus. This picture is reinforced by our isophotal analysis where complex radial dependencies of ellipticity, position angle, and isophotal shape parameter a₄ are exhibited not only by objects like IC 3328, for which the presence of a disk component has been confirmed, but by many apparently normal dEs as well. In addition, we find a relation between the effective surface brightness, at a given luminosity, and the strength of the offset of the galaxy's nucleus with respect to the center of the isophotes. Dwarfs with large nuclear offsets also tend to have stronger isophotal twists. However, such twists are preferentially found in apparently round (ε < 0.3) galaxies and are always accompanied by significant radial changes of the ellipticity, which clearly points to a projection effect. In sum, our findings suggest the presence of substructure in most, and preferentially in the less compact, bright early-type dwarfs. The physical (dynamical) meaning of this has yet to be explored.