Spiral galaxies with WFPC2. III. Nuclear cusp slopes

In this paper, the third of a series dedicated to the investigation of the nuclear properties of spiral galaxies, we (1) model the Wide Field Planetary Camera 2 F606W nuclear surface brightness profiles of 41 spiral galaxies presented in Papers I and II with the analytic law introduced by Lauer et al. and (2) deconvolve these surface brightness profiles and their analytic 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) comparatively shallower stellar cusps, and likely lower nuclear densities, than R-1/4 law bulges.