Evidence for a New Elliptical-Galaxy Paradigm: Srsic and Core Galaxies

We fit the surface-brightness profiles of 21 elliptical galaxies using boththe Sersic function and a new empirical model which combines an inner power lawwith an outer Sersic function. The profiles are combinations of deconvolved HSTprofiles from the literature and ellipse fits to the full WFPC2 mosaic images,and thus span a radial range from ~ 0.02 arcsec to ~ twice the half-lightradius. We are able to accurately fit the entire profiles using either theSersic function or our new model. In doing so, we demonstrate that most, if notall, so-called ``power-law'' galaxies are better described as ``Sersicgalaxies'' -- they are well modeled by the three-parameter Sersic profile intothe limits of HST resolution -- and that ``core'' galaxies are best understoodas consisting of an outer Sersic profile with an inner power-law cusp, which isa downward deviation from the inward extrapolation of the Sersic profile. Thisdefinition of cores resolves ambiguities that result when the popular ``Nukerlaw'' is fitted to the profiles of ellipticals and bulges, particularly atlower luminosities. We also find that using the Nuker law to model core-galaxynuclear profiles systematically overestimates the core radii by factors of1.5--4.5 and underestimates the inner power-law slope by ~ 20--40% or more.