We have analysed deep R -band images , down to a limiting surface brightness of 26.5 R-mag arcsec ^ { -2 } ( equivalent to \sim 28 B-mag arcsec ^ { -2 } ) , of 5 cD galaxies to determine the shape of the surface brightness profiles of their extended stellar envelopes .
Both de Vaucouleurs R ^ { 1 / 4 } model and Sérsic ’ s R ^ { 1 / n } model , 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 Sérsic functions to measure the shape of the inner and outer components of the cD galaxies .
We show that , for 3 out of our 5 galaxies , the surface brightness profiles are best fit by an inner Sérsic model , with indices n \sim 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 to 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 .