The Structure of Nuclear Star Clusters in Nearby Late-type Spiral
  Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging
  We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of
ten of the nearest and brightest nuclear clusters residing in late-type spiral
galaxies, in seven bands that span the near-ultraviolet to the near-infrared.
Structural properties of the clusters were measured by fitting two-dimensional
surface brightness profiles to the images using GALFIT. The clusters exhibit a
wide range of structural properties. For six of the ten clusters in our sample,
we find changes in the effective radius with wavelength, suggesting radially
varying stellar populations. In four of the objects, the effective radius
increases with wavelength, indicating the presence of a younger population
which is more concentrated than the bulk of the stars in the cluster. However,
we find a general decrease in effective radius with wavelength in two of the
objects in our sample, which may indicate extended, circumnuclear star
formation. We also find a general trend of increasing roundness of the clusters
at longer wavelengths, as well as a correlation between the axis ratios of the
NCs and their host galaxies. These observations indicate that blue disks
aligned with the host galaxy plane are a common feature of nuclear clusters in
late-type galaxies, but are difficult to detect in galaxies that are close to
face-on. In color-color diagrams spanning the near-UV through the near-IR, most
of the clusters lie far from single-burst evolutionary tracks, showing evidence
for multi-age populations. Most of the clusters have integrated colors
consistent with a mix of an old population (> 1 Gyr) and a young population
(~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity
to populations with a mix of ages that would not be possible to achieve with
imaging in optical bands only.
