Spiral Galaxies with WFPC2. II. The Nuclear Properties of 40 Objects

We report the analysis of HST WFPC2 F606W images of 40 spirals belonging to the sample introduced in Carollo et al. (paper I), where 35 other targets were discussed. We describe the optical morphology of the new 40 galaxies, derive the surface brightness profiles for 25 of them, and present the results of photometric decompositions of these profiles into a ``bulge'' (R¹/4 or exponential) and a disk component. The analysis of the sample of 75 galaxies puts on a statistically more solid ground the main results presented in paper I. Furthermore, the analysis of the sample of 75 objects shows that: A. Several of the non-classical inner structures are well fitted by an exponential profile. These ``exponential bulges'' are fainter than R¹/4 bulges for given total galaxy luminosity and (catalog) Hubble type later than Sab. B. Irregular/exponential bulges typically host central compact sources. C. The central sources are present in all types of disk galaxies, starting with systems as early as S0a. About 60 % of Sb to Sc galaxies host a central compact source. Many of the galaxies which host compact sources contain a barred structure. D. Galaxies with apparent nuclear star formation and the brightest compact sources, are preferentially the early- and intermediate- type (S0a-Sb) systems. E. None of the features depends on environment: isolated and not isolated galaxies show indistinguishable properties. Independently from the physical nature of the non-classical inner structures, our main conclusion is that a significant fraction of galaxies classified from the ground as relatively early-type shows a rich variety of central properties, and little or no morphological/photometric evidence for a smooth, R¹/4-law bulge.