WFPC2 Observations of Compact Star Cluster Nuclei in Low-Luminosity Spiral Galaxies

We have used the Wide Field Planetary Camera 2 aboard the Hubble SpaceTelescope to image the compact star cluster nuclei of the nearby, late-type,low-luminosity spiral galaxies NGC 4395, NGC 4242, and ESO 359-029. We alsoanalyze archival WFPC2 observations of the compact star cluster nucleus of M33.A comparative analysis of the structural and photometric properties of thesefour nuclei is presented. All of the nuclei are very compact, with luminositydensities increasing at small radii to the resolution limit of our data. NGC4395 contains a Seyfert 1 nucleus with a distinct bipolar structure and brightassociated filaments which are likely due to [OIII] emission. The M33 nucleushas a complex structure, with elongated isophotes and possible signatures ofweak activity, including a jet-like component. The other two nuclei are notknown to be active, but share similar physical size scales and luminosities tothe M33 and NGC 4395 nuclei. The circumnuclear environments of all four of ourprogram galaxies are extremely diffuse, have only low-to-moderate starformation, and appear to be devoid of large quantities of dust. The centralgravitational potentials of the galaxies are also quite shallow, making theorigin of these types of `naked' nuclei problematic.Comment: to appear in the July 1999 Astronomical Journal; 38 pages (Latex), 5 tables (postscript), 21 figures (gif); postscript versions of the figures may be obtained via anonymous ftp at ftp://ftp.cv.nrao.edu/NRAO-staff/lmatthew/lanl-nucle