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

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Abstract
We report the analysis of Hubble Space Telescope Wide Field Planetary Camera 2 F606W images of 40 spiral galaxies belonging to the sample introduced in Paper I, where 35 other targets were discussed. We describe the optical morphological properties 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 enlarged sample of 75 galaxies puts on a statistically more solid ground the main results presented in Paper I: (1) In ≈30% of the galaxies, the inner, morphologically distinct structures have an irregular appearance. Some of these "irregular bulges" are likely to be currently forming stars. (2) Resolved, central compact sources are detected in about 50% of the galaxies. (3) The central compact sources in galaxies with nuclear star formation are brighter, for similar sizes, than those in non–star-forming galaxies. (4) The luminosity of the compact 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 an exponential profile. These "exponential bulges" are typically fainter than R1/4 bulges, for a 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 that host compact sources contain a barred structure. (d) Galaxies with apparent nuclear star formation, which also host the brightest compact sources, are preferentially the early- and intermediate-type (S0a–Sb) systems. (e) None of the features depend on environment: isolated and nonisolated galaxies show indistinguishable properties. Independent of the physical nature of the nonclassical inner structures, our main conclusion is that a significant fraction of galaxies classified from the ground as relatively early-type spirals show a rich variety of central properties and little or no morphological/photometric evidence for a smooth, R1/4 law bulge.
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