The Centers of Early‐Type Galaxies withHubble Space Telescope. VI. Bimodal Central Surface Brightness Profiles

We combine the results from several HST investigations of the centralstructure of early-type galaxies to generate a large sample of parameterizedsurface photometry. The studies included were those that used the "Nuker law"to characterize the inner light distributions of the galaxies. The samplecomprises WFPC1 and WFPC2 V band observations published earlier by our group, Rband WFPC2 photometry of Rest et al., NICMOS H band photometry by Ravindranathet al. and Quillen et al., and the BCG WFPC2 I band photometry of Laine et al.The distribution of the logarithmic slopes of the central profiles stronglyaffirms that the central structure of elliptical galaxies with Mv < -19 isbimodal, based on both parametric and non-parametric analysis. At the HSTresolution limit, most galaxies are either power-law systems, which have steepcusps in surface brightness, or core systems, which have shallow cusps interiorto a steeper envelope brightness distribution. A rapid transition between thetwo forms occurs over the luminosity range -22 < Mv < -20, with coresdominating at the highest luminosities, and power-laws at the lowest. There area few "intermediate" systems that have both cusp slopes and total luminositiesthat fall within the core/power-law transition, but they are rare and do notfill in the overall bimodal distribution of cusp slopes. These results areinconsistent with the Ferrarese et al. Virgo Cluster Survey (VCS) analysis.However, using galaxies common to the VCS samples, we demonstrate that the VCSmodels of the cusps are either a poor match to the observations or consist offorms fitted to the galaxy envelopes and extrapolated inward to the HSTresolution limit.Comment: Submitted to the Astrophysical Journal; 60 pages and 20 postscript figure