SPIRAL GALAXIES WITH WFPC2 .1. NUCLEAR MORPHOLOGY, BULGES, STAR-CLUSTERS, AND SURFACE BRIGHTNESS PROFILES

We report the analysis of HST WFPC2 F606W images of a sample of 35 spi ral galaxies. We describe the optical morphological properties of the galaxies on scales of a few tens of parsecs, derive the surface bright ness profiles for LX of them, and present the results of analytical fi ts to these profiles. We find that: (I) Several galaxies, despite thei r classification as relatively early spirals, show little or no morpho logical evidence for a smooth, featureless (i.e., classical) bulge. Fu rthermore, even in the classical bulges, nuclear dust lanes are detect ed, similar to what is observed in early-type galaxies. (II) In severa l cases spiral structure reaches down to the innermost accessible scal es. (III) A central, bright component is often identified, However, th is does not resemble a classical bulge but has instead a highly irregu lar morphology. (IV) Bright knots, very likely star forming regions, a re often present within the inner galactic regions in these irregular bulges, In several other cases, nuclear star formation is observed, bu t it is unclear whether this is associated with a small irregular bulg e or with the inner disk. (V) Resolved, central compact sources are fo und in 18 of the 35 galaxies, including several early-type spirals. In two objects, NGC 1483 and NGC 3259, the central source is unresolved. Within the errors and the limited statistics, the central compact sou rce luminosity seems independent of Hubble type, but correlates with t he luminosity, of the disk galaxy. (VI) The central compact sources in star forming galaxies are typically brighter, for similar radii, than those in non star forming galaxies. (VII) The brightest compact sourc es have properties similar to those of young star clusters in the abso lute magnitude (M-V) versus half-light radius (R-e) plane. The faintes t compact sources are bracketed, in the MV-R, plane, between Kormendy' s fit to elliptical galaxies and the correlation defined by classical R-1/4 bulges and Galactic globular clusters. The latter implies a roug hly constant mean surface brightness within the effective radius, Ther efore, the luminosity sequence of the compact sources might represent an age sequence of superluminous star clusters in the centers of disk galaxies. The widespread presence of star formation in the irregular b ulges, the irregular bulges themselves, and the presence of superlumin ous star clusters in the nuclei of spirals, support scenarios in which a fraction of bulges forms relatively late, in dissipative accretion events driven by the disk. (C) 1997 American Astronomical Society. [S0 004-6256(97)00912-6].