Photometric Properties and Scaling Relations of Early Type Brightest Cluster Galaxies

Abstract

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 redshift less than 0.1. We perform accurate background subtractions, and surface photometry for these BCGs to 25 $\magsec$ 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 halos that are likely 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 ($R \propto L^\alpha$) and Faber-Jackson ($L \propto \sigma^\beta$) relations than the bulk of early type galaxies. Furthermore, the power-law indices ($\alpha$ and $\beta$) in these relations increase as the isophotal limits become deeper. For isophotal limits from 22 to 25 $\magsec$, BCGs are usually larger than the bulk of early type galaxies, and a large fraction ($\sim 49%$) of BCGs have disky 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.