The ACS Fornax Cluster Survey. II. The central brightness profiles of early-type galaxies: A characteristic radius on nuclear scales and the transition from central luminosity deficit to excess

We analyze brightness profiles for 143 early-type galaxies in the Virgo and Fornax Clusters, observed with the Advanced Camera for Surveys on the Hubble Space Telescope. Sersic models are found to provide accurate representations of the global profiles with a notable exception: the observed profiles deviate systematically inside a characteristic break'' radius of Rb approximate to 0.02 +0.025 -0.01 R-e, where R-e is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies (M-B less than or similar to - 20) typically show central light deficits with respect to the inward extrapolation of the Sersic model, while the great majority of low-and intermediate-luminosity galaxies (-19.5 less than or similar to M-B less than or similar to - 15) show central light excesses; galaxies of intermediate luminosities (-20 less than or similar to M-B less than or similar to - 19.5) are generally well fitted by Sersic models over all radii. We show that the slope, gamma', of the central surface brightness profiles, when measured at fixed fractions of Re, varies smoothly as a function of galaxy luminosity in a manner that depends sensitively on the choice of measurement radius. We find no evidence for a core/power-law dichotomy, and show that a recent claim of strong bimodality in gamma' is likely an artifact of the biased galaxy selection function used in that study. To provide a more robust characterization of the inner regions of galaxies, we introduce a parameter Delta(0.02) = log (L-g/L-S)-where L-g and L-S are the integrated luminosities inside 0.02R(e) of the observed profile and of the inward extrapolation of the outer Sersic model-to describe the central luminosity deficit (Delta(0.02) < 0) or excess (Delta(0.02) > 0). We find that Delta(0.02) varies smoothly over the range of approximate to 720 in luminosity spanned by the sample galaxies, with again no evidence for a dichotomy. We argue that the central light excesses in M-B greater than or similar to - 19 galaxies may be the analogs of the dense central cores predicted by some numerical simulations to form via gas inflows.