Photometric properties and origin of bulges in SB0 galaxies

Abstract

We have derived the photometric parameters for the structural components of a sample of fourteen SB0 galaxies by applying a parametric photometric decomposition to their observed $I$-band surface brightness distribution. We find that SB0 bulges are similar to bulges of the early-type unbarred spirals, i.e. they have nearly exponential surface brightness profiles ($ =1.48\pm0.16$) and their effective radii are strongly coupled to the scale lengths of their surrounding discs ($ =0.20\pm0.01$). The photometric analysis alone does not allow us to differentiate SB0 bulges from unbarred S0 ones. However, three sample bulges have disc properties typical of pseudobulges. The bulges of NGC 1308 and NGC 4340 rotate faster than bulges of unbarred galaxies and models of isotropic oblate spheroids with equal ellipticity. The bulge of IC 874 has a velocity dispersion lower than expected from the Faber--Jackson correlation and the fundamental plane of the elliptical galaxies and S0 bulges. The remaining sample bulges are classical bulges, and are kinematically similar to lower-luminosity ellipticals. In particular, they follow the Faber--Jackson correlation, lie on the fundamental plane and those for which stellar kinematics are available rotate as fast as the bulges of unbarred galaxies.