SPIRAL GALAXIES WITH WFPC2 - II - THE NUCLEAR PROPERTIES OF 40 OBJECTS

We report the analysis of Hubble Space Telescope Wide Field Planetary Camera 2 F606W images of 40 spiral galaxies belonging to the sample in troduced in Paper I, where 35 other targets were discussed. We describ e the optical morphological properties of the new 40 galaxies, derive the surface brightness profiles for 25 of them, and present the result s of photometric decompositions of these profiles into a ''bulge'' (R- 1/4 or exponential) and a disk component. The analysis of the enlarged sample of 75 galaxies puts on a statistically more solid ground the m ain results presented in Paper I: (1) In approximate to 30% of the gal axies, the inner, morphologically distinct structures have an irregula r appearance. Some of these ''irregular bulges'' are likely to be curr ently forming stars. (2) Resolved, central compact sources are detecte d in about 50% of the galaxies. (3) The central compact sources in gal axies with nuclear star formation are brighter, for similar sizes, tha n those in non-star-forming galaxies. (4) The luminosity of the compac t sources correlates with the total galactic luminosity. Furthermore, the analysis of the enlarged sample of 75 objects shows the following: (a) Several of the nonclassical inner structures are well fitted by a n exponential profile. These ''exponential bulges'' are typically fain ter than R-1/4 bulges, for a given total galaxy luminosity and (catalo g) Hubble type later than Sab. (b) Irregular/exponential bulges typica lly 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 that host compact sources contain a barred structure. (d) Galaxies with apparent nuclear star formation, which also host the br ightest compact sources, are preferentially the early- and intermediat e-type (S0a-Sb) systems. (e) None of the features depend on environmen t: isolated and nonisolated galaxies show indistinguishable properties . Independent of the physical nature of the nonclassical inner structu res, our main conclusion is that a significant fraction of galaxies cl assified from the ground as relatively early-type spirals show a rich variety of central properties and little or no morphological/photometr ic evidence for a smooth, R-1/4 law bulge.