Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description

We have analysed deep R-band images, down to a limiting surface brightness of 26.5 R-mag arcsec(-2) (equivalent to similar to 28 B-mag arcsec(-2)), of five cD galaxies to determine the shape of the surface brightness profiles of their extended stellar envelopes. Both de Vaucouleurs R-1/4 and Sersic R-1/n models, on their own, provide a poor description of the surface brightness profiles of cD galaxies. This is due to the presence of outer stellar envelopes, thought to have accumulated over the merger history of the central cluster galaxy and also from the tidal stripping of galaxies at larger cluster radii. We therefore simultaneously fit two Sersic functions to measure the shape of the inner and outer components of the cD galaxies. We show that, for three out of our five galaxies, the surface brightness profiles are best fitted by an inner Sersic model, with indices n similar to 1-6, and an outer exponential component. For these systems, the galaxy-to-envelope size ratio is 0.1-0.4 and the contribution of the stellar envelope to the total R-band light (i.e. galaxy + envelope) is around 60-80 per cent (based on extrapolation to a 300 kpc radius). The exceptions are NGC 6173, for which our surface brightness profile modelling is consistent with just a single component (i.e. no envelope) and NGC 4874 which appears to have an envelope with a de Vaucouleurs, rather than exponential, profile.