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We analyse HST surface brightness profiles for 143 early-type galaxies in theVirgo and Fornax Clusters. Sersic models provide accurate descriptions of theglobal profiles with a notable exception: the observed profiles deviatesystematically inside a characteristic "break" radius of R_b ~ 0.02R_e whereR_e is the effective radius of the galaxy. The sense of the deviation is suchthat bright galaxies (M_B < -20) typically show central light deficits withrespect to the inward extrapolation of the Sersic model, while the greatmajority of low- and intermediate-luminosity galaxies (-19.5 < M_B < -15) showcentral light excesses; galaxies occupying a narrow range of intermediateluminosities (-20 < M_B < -19.5) are usually well fitted by Sersic models overall radii. The slopes of the central surface brightness profiles, when measuredat fixed fractions of R_e, vary smoothly as a function of galaxy luminosity ina manner that depends sensitively on the choice of measurement radius. We showthat a recent claim of strong bimodality in slope is likely an artifact of thegalaxy selection function used in that study. To provide a more robustcharacterization of the inner regions of galaxies, we introduce a parameterthat describes the central luminosity deficit or excess relative to the inwardextrapolation of the outer Sersic model. We find that this parameter variessmoothly over the range of ~ 720 in blue luminosity spanned by the Virgo andFornax sample galaxies, with no evidence for a dichotomy. We argue that thecentral light excesses (nuclei) in M_B > -19 galaxies may be the analogs of thedense central cores that are predicted by some numerical simulations to formvia gas inflows. (ABRIDGED)

astrophysical journal/the astrophysical journal

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