A New Empirical Model for the Structural Analysis of Early-Type Galaxies, and A Critical Review of the Nuker Model

The Nuker law was designed to match the inner few (~3-10) arcseconds ofpredominantly nearby (< 30 Mpc) early-type galaxy light-profiles; it was neverintended to describe an entire profile. The Sersic model, on the other hand,was developed to fit the entire profile; however, due to the presence ofpartially depleted galaxy cores, the Sersic model cannot always describe thevery inner region. We have therefore developed a new empirical model consistingof an inner power-law, a transition region, and an outer Sersic model toconnect the inner and outer structure of elliptical galaxies. Moreover, thestability of the Nuker model parameters are investigated. Surprisingly, noneare found to be stable quantities; all are shown to vary systematically with aprofile's fitted radial extent, and often by more than 100%. Consideringelliptical galaxies spanning a range of 7.5 magnitudes, the central stellardensity of the underlying host galaxy is observed to increase with galaxyluminosity until the onset of core formation, detected only in the brightestelliptical galaxies. We suggest that the so-called ``power-law'' galaxies mayactually be described by the Sersic model over their entire radial range