Spiral galaxies with WFPC2: III. Nuclear Cusp Slopes

Abstract

In this paper, the third of a series dedicated to the investigation of the nuclear properties of spiral galaxies, we have (i) modelled the WFPC2 F606W nuclear surface brightness profiles of 41 spiral galaxies presented in Carollo et al. 1997c, 1998 with the analytical law introduced by Lauer et al. 1995, and (ii) deprojected these surface brightness profiles and their analytical fits, so as to estimate the nuclear stellar densities of bulges of spiral galaxies. We find that the nuclear stellar cusps (quantified by the average logarithmic slope of the surface brightness profiles within 0.1''-0.5'') are significantly different for R^1/4-law and exponential bulges. The former have nuclear properties similar to those of early-type galaxies, i.e. similar values of nuclear cusps for comparable luminosities, and increasingly steeper stellar cusps with decreasing luminosity. By contrast, exponential bulges have (underlying the light contribution from photometrically distinct, central compact sources) comparative shallower stellar cusps, and likely lower nuclear densities, than R^1/4-law bulges.