The Luminosity Profiles of Brightest Cluster Galaxies

We have derived detailed R-band luminosity profiles and structural parameters for a total of 430 brightest cluster galaxies (BCGs), down to a limiting surface brightness of 24.5 mag arcsec^-2. Light profiles were initially fitted with a Sérsic's R ^1/n model, but we found that 205 (~48%) BCGs require a double component model to accurately match their light profiles. The best fit for these 205 galaxies is an inner Sérsic model, with indices n ~ 1-7, plus an outer exponential component. Thus, we establish the existence of two categories of the BCG luminosity profiles: single and double component profiles. We found that double profile BCGs are brighter (~0.2 mag) than single profile BCGs. In fact, the Kolmogorov-Smirnov test applied to these subsamples indicates that they have different total magnitude distributions, with mean values M_R = -23.8 ± 0.6 mag for single profile BCGs and M_R = -24.0 ± 0.5 mag for double profile BCGs. We find that partial luminosities for both subsamples are indistinguishable up to r = 15 kpc, while for r > 20 kpc the luminosities we obtain are on average 0.2 mag brighter for double profile BCGs. This result indicates that extra-light for double profile BCGs does not come from the inner region but from the outer regions of these galaxies. The best-fit slope of the Kormendy relation for the whole sample is a = 3.13 ± 0.04. However, when fitted separately, single and double profile BCGs show different slopes: a _single = 3.29 ± 0.06 and a _double = 2.79 ± 0.08. Also, the logarithmic slope of the metric luminosity α is higher in double profile BCGs (α_double = 0.65 ± 0.12) than in single profile BCGs (α_single = 0.59 ± 0.14). The mean isophote outer ellipticity (calculated at μ ~ 24 mag arcsec^-2) is higher in double profile BCGs (e _double = 0.30 ± 0.10) than in single profile BCGs (e _single = 0.26 ± 0.11). Similarly, the mean absolute value of inner minus outer ellipticity is also higher in double profile BCGs. From a subsample of 24 BCGs, we found strong evidence that extra-light at intermediate radii in double profile BCGs is related to the presence of a faint stellar envelope. Similarly, from another subsample of 12 BCGs we also found that extra-light is related to star formation. On the other hand, we did not find differences between these two BCG categories when we compared global cluster properties such as the BCG-projected position relative to the cluster X-ray center emission, X-ray luminosity, or BCG orientation with respect to the cluster position angle.