THE CENTERS OF EARLY-TYPE GALAXIES WITH HST .2. EMPIRICAL-MODELS AND STRUCTURAL PARAMETERS

We present a set of structural parameters for the central parts of 57 early-type galaxies observed with the Planetary Camera of the Hubble S pace Telescope. These parameters are based on a new empirical law that successfully characterizes the centers of early-type galaxies. This e mpirical law assumes that the surface brightness profile is a combinat ion of two power laws with different slopes gamma and beta for the inn er and outer regions. Conventional structural parameters such as core radius and central surface brightness are replaced by break radius r(b ), where the transition between power-law slopes takes place, and surf ace brightness mu(b) at that radius. An additional parameter alpha des cribes the sharpness of the break. The structural parameters are deriv ed using a chi(2) minimization process applied to the mean surface bri ghtness profiles. The resulting model profiles generally give very goo d agreement to the observed profiles out to the radius of similar to 1 0 '' imaged by the Planetary Camera. Exceptions include galaxies which depart from pure power laws at large radius, those with strong nuclea r components, and galaxies partly obscured by dust. The uncertainties in the derived parameters are estimated using Monte Carlo simulations which test the stability of solutions in the face of photon noise and the effects of the deconvolution process. The covariance of the struct ural parameters is examined by computing contours of constant chi(2) i n multi-dimensional parameter space. (C) 1996 American Astronomical So ciety.