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UV imaging permits investigation of extreme Population I material in the circumnuclear regions of galaxies ($r < $ a few kpc). We present a comparison of circumnuclear star formation in four disk galaxies observed by the Ultraviolet Imaging Telescope (UIT): NGC~4321 (M100), NGC~1068, NGC~3031 (M81), and NGC~1317. UIT obtained data in a near-UV ($\lambda\lambda\ 2490$ \AA, $\Delta\lambda\ 1150$\ \AA), and a far-UV ($\lambda\lambda\ 1520$ \AA, $\Delta\lambda\ 354$\ \AA) bandpass with a resolution of 2-3\,$^{\prime\prime}$.
In both M100 and NGC~1317 the primary feature seen at UV wavelengths is a broken elliptical ring of bright knots approximately centered on the optically-defined nucleus. The UV colors of these knots are characteristic of OB stars. Diffuse emission is observed between the knots, and the nucleus lies at a minimum in the UV surface brightness. In NGC~1317, this structure is $\sim 26^{\prime\prime} \times 19^{\prime\prime}$ in extent ($ 3.1 \times 2.3$ kpc), in M100 $\sim 10^{\prime\prime} \times 6^{\prime\prime}$ (1.5 $\times$ 0.9 kpc); their total far-UV luminosities are comparable.
NGC~1068 also exhibits unusually high star formation activity in its near-nuclear zone (Neff et~al. ApJ, submitted). Several luminous knots are observed within $\sim 3$ kpc embedded in a strong extended UV background which pervades both the arm and interarm regions and which extends to greater radii than the knot population.
M81, by contrast, shows no evidence of recent high-mass star formation within 3~kpc of its nucleus. All massive star formation appears confined to the well-defined spiral arms. The UV luminosity and color of the nuclear source is consistent with the expected properties of the UVX component from the old bulge population (O'Connell et~al. 1992, ApJ 395, L45).
We use the UIT imagery to determine the spatial distribution and rate of massive star formation in these systems.
