Photometric properties and scaling relations of early-type Brightest Cluster Galaxies

We investigate the photometric properties of the early-type Brightest Cluster Galaxies (BCGs) using a carefully selected sample of 85 BCGs from the C4 cluster catalogue with a redshift of less than 0.1. We perform accurate background subtractions and surface photometry for these BCGs to 25 mag arcsec(-2) in the Sloan r band. By quantitatively analysing the gradient of the Petrosian profiles of BCGs, we find that a large fraction of BCGs have extended stellar envelopes in their outskirts; more luminous BCGs tend to have more extended stellar haloes that are likely to be connected with mergers. A comparison sample of elliptical galaxies was chosen with similar apparent magnitude and redshift ranges, for which the same photometric analysis procedure is applied. We find that BCGs have steeper size-luminosity (R proportional to L-alpha) and Faber-Jackson (L proportional to sigma(beta)) relations than the bulk of early-type galaxies. Furthermore, the power-law indices (a and,B) in these relations increase as the isophotal limits become deeper. For isophotal limits from 22 to 25 mag arcsec(-2), BCGs are usually larger than the bulk of early-type galaxies, and a large fraction (similar to 49 per cent) of BCGs have discy isophotal shapes. The differences in the scaling relations are consistent with a scenario where the dynamical structure and formation route of BCGs may be different from the bulk of early-type galaxies; in particular dry (dissipationless) mergers may play a more important role in their formation. We highlight several possible dry merger candidates in our sample.