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 analyse 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. S´ersic 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 ≈ 0.02+0.025 −0.01 Re, where Re is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies (MB . −20) typically show central light deficits with respect to the inward extrapolation of the S´ersic model, while the great majority of low- and intermediate-luminosity galaxies (−19.5 . MB . −15) show central light excesses; galaxies of intermediate luminosities (−20 . MB . −19.5) are generally well fitted by S´ersic models over all radii. We show that the slope, ′, 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 ′ 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, 0.02 = log (Lg/Ls) — where Lg and Ls are the integrated luminosities inside 0.02Re of the observed profile and of the inward extrapolation of the outer S´ersic model — to describe the central luminosity deficit ( 0.02 \u3c 0) or excess ( 0.02 \u3e 0). We find that 0.02 varies smoothly over the range of ≈ 720 in luminosity spanned by the sample galaxies, with again no evidence for a dichotomy. We argue that the central light excesses in MB & −19 galaxies may be the analogs of the dense central cores predicted by some numerical simulations to form via gas inflows