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

Abstract

We analyse HST surface brightness profiles for 143 early-type galaxies in the Virgo and Fornax Clusters. Sersic models provide accurate descriptions of the global profiles with a notable exception: the observed profiles deviate systematically inside a characteristic "break" radius of R_b ~ 0.02R_e where R_e is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies (M_B < -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 < M_B < -15) show central light excesses; galaxies occupying a narrow range of intermediate luminosities (-20 < M_B < -19.5) are usually well fitted by Sersic models over all radii. The slopes of the central surface brightness profiles, when measured at fixed fractions of R_e, vary smoothly as a function of galaxy luminosity in a manner that depends sensitively on the choice of measurement radius. We show that a recent claim of strong bimodality in slope is likely an artifact of the galaxy selection function used in that study. To provide a more robust characterization of the inner regions of galaxies, we introduce a parameter that describes the central luminosity deficit or excess relative to the inward extrapolation of the outer Sersic model. We find that this parameter varies smoothly over the range of ~ 720 in blue luminosity spanned by the Virgo and Fornax sample galaxies, with no evidence for a dichotomy. We argue that the central light excesses (nuclei) in M_B > -19 galaxies may be the analogs of the dense central cores that are predicted by some numerical simulations to form via gas inflows. (ABRIDGED)

To appear in the Astrophysical Journal, December 2007. Full resolution paper available at http://www.cfa.harvard.edu/~ajordan/ACSFCS/Home.html

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