The Luminosity Profiles of Brightest Cluster Galaxies

Abstract

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 Sersic’s ´ R1/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 Sersic 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 MR = −23.8 ± 0.6 mag for single profile BCGs and MR = −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: asingle = 3.29 ± 0.06 and adouble = 2.79 ± 0.08. Also, the logarithmic slope of the metric luminosity α is higher in double profile BCGs (αdouble = 0.65±0.12) than in single profile BCGs (αsingle = 0.59±0.14). The mean isophote outer ellipticity (calculated atμ ∼ 24 mag arcsec−2) is higher in double profile BCGs (edouble = 0.30 ± 0.10) than in single profile BCGs (esingle = 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.