STScI Preprint #1231

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

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS5-26555.

We report the analysis of HST WFPC2 F606W images of 40 spiral galaxies belonging to the sample introduced in Carollo et al. (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" (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 main results presented in paper I, namely: I.  In 30% of the galaxies, the inner, morphologically-distinct, structures have an irregular appearance. Some of these "irregular bulges" are likely to be currently star forming. II. Resolved, central compact sources are detected in about 50% of the galaxies. III. The central compact sources in galaxies with nuclear star formation are brighter, for similar sizes, than those in non star forming galaxies. IV. The luminosity of the compact sources correlates with the total galactic luminosity.

Furthermore, the analysis of the enlarged 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 R^1/4 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, which also host 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 spirals shows a rich variety of central properties, and little or no morphological/photometric evidence for a smooth, R^1/4-law bulge.

Status:

Appeared in: The Astronomical Journal, 116:68-84, 1998

Affiliations:

¹)Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²) Hubble Fellow.
³) On assignment from the Space Science Dept. of the European Space Agency.
⁴) Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218.
⁵) On leave from the Scuola Normale Superiore, Piazza dei Cavalieri 7, I56126 Pisa, Italy.