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∝Lα) and Faber–Jackson (L∝σβ) relations than the bulk of early-type galaxies. Furthermore, the power-law indices (α and β) 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 (∼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.