WFPC2 observations of compact star cluster nuclei in low-luminosity spiral galaxies

We have used the Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope to obtain F450W and F814W (B- and I-band) observations of the compact star cluster nuclei of the nearby, late-type, low-luminosity spiral galaxies NGC 4395, NGC 4242, and ESO 359-029. In addition, we analyze archival WFPC2 observations of the compact star cluster nucleus of M33. All of these galaxies are structurally diffuse, with moderately low surface brightnesses and little or no discernible bulge component. Here we present a comparative analysis of the structural and photometric properties of their nuclei. NGC 4395 contains a Seyfert 1 nucleus; M33 has some signatures of weak nuclear activity; the other two galaxies are not known to be active. All of the nuclei have M-I similar to -11; hence these represent a little explored low-luminosity extension of the galactic nuclear activity sequence in a class of host galaxy not traditionally associated with galactic nuclear phenomena. These kinds of compact nuclei appear to be quite common in low luminosity, late-type spirals. Our Planetary Camera 2 images partially resolve the nuclei of all four galaxies. A simple model consisting of an isothermal sphere plus a point source provides a good model for the observed radial intensity distribution in all cases and permits an exploration of the underlying nuclear structures and spatial scales. Despite their low luminosities, all of the nuclei are very compact. In all cases the luminosity densities are increasing at small radii to the resolution limit of our data. In spite of having similar size scales and luminosities, the nuclei in our sample span a wide range of B-I color. This may be a signature of different evolutionary phases. The M33 nucleus exhibits complex structure; its isophotes are elongated, and it has a possible jetlike component. The Seyfert nucleus of NGC 4395 has an extremely blue color (B-I = -0.16) and is the most structurally complex nucleus in our sample. Circularly symmetric fits to its underlying structure reveal a distinct bipolar pattern. A pair of bright filaments located on one side of the nucleus are probably due to [O III] emission from gas within a nuclear ionization cone. NGC 4395 appears to contain an underlying normal star cluster nucleus that is hosting activity. NGC 4242 shows evidence of a slightly elongated, bar-shaped feature at its center. The ESO 359-029 nucleus appears relatively symmetric and featureless at the resolution limit of our data, but it is clearly very compact. The circumnuclear environments of all four of our program galaxies are extremely diffuse, have only low to moderate star formation, and appear to be devoid of large quantities of dust. The central gravitational potentials of the galaxies are also quite shallow making the origin of these types of "naked" nuclei problematic.