Nuclear star clusters in 228 spiral galaxies in the HST/WFPC2 archive:
  catalogue and comparison to other stellar systems
  We present a catalogue of photometric and structural properties of 228
nuclear star clusters (NSCs) in nearby late-type disk galaxies. These new
measurements are derived from a homogeneous analysis of all suitable WFPC2
images in the $HST$ archive. The luminosity and size of each NSC is derived
from an iterative PSF-fitting technique, which adapts the fitting area to the
effective radius ($r_{\rm eff}$) of the NSC, and uses a WFPC2-specific PSF
model tailored to the position of each NSC on the detector. The luminosities of
NSCs are $\leq\!10^8L_{\rm V,\odot}$, and their integrated optical colours
suggest a wide spread in age. We confirm that most NSCs have sizes similar to
Globular Clusters (GCs), but find that the largest and brightest NSCs occupy
the regime between Ultra Compact Dwarf (UCD) and the nuclei of early-type
galaxies in the size-luminosity plane. The overlap in size, mass, and colour
between the different incarnations of compact stellar systems provides a
support for the notion that at least some UCDs and the most massive Galactic
GCs, may be remnant nuclei of disrupted disk galaxies. We find tentative
evidence for the NSCs' $r_{\rm eff}$ to be smaller when measured in bluer
filters, and discuss possible implications of this result. We also highlight a
few examples of complex nuclear morphologies, including double nuclei, extended
stellar structures, and nuclear $F606W$ excess from either recent
(circum-)nuclear star formation and/or a weak AGN. Such examples may serve as
case studies for ongoing NSC evolution via the two main suggested mechanisms,
namely cluster merging and $in\ situ$ star formation.
