Bright, Relatively Isolated Star Clusters in PHANGS-HST Galaxies:
  Aperture Corrections, Quantitative Morphologies, and Comparison with
  Synthetic Stellar Population Models
  Using PHANGS-HST NUV-U-B-V-I imaging of 17 nearby spiral galaxies, we study
samples of star clusters and stellar associations, visually selected to be
bright and relatively isolated, for three purposes: to compute aperture
corrections for star cluster photometry, to explore the utility of quantitative
morphologies in the analysis of clusters and associations, and to compare to
synthetic stellar population models. We provide a technical summary of our
procedures to determine aperture corrections, a standard step in the production
of star cluster candidate catalogs, and compare to prior work. We also use this
specialized sample to launch an analysis into the measurement of star cluster
light profiles. We focus on one measure, $M_{20}$ (normalized second order
moment of the brightest 20% of pixels), applied previously to study the
morphologies of galaxies. We find that $M_{20}$ in combination with UB-VI
colors, yields a parameter space where distinct loci are formed by
single-peaked symmetric clusters, single-peaked asymmetric clusters, and
multi-peaked associations. We discuss the potential applications for using
$M_{20}$ to gain insight into the formation and evolution of clusters and
associations. Finally, we compare the color distributions of this sample with
various synthetic stellar population models. One finding is that the standard
procedure of using a single-metallicity SSP track to fit the entire population
of clusters in a given galaxy should be revisited, as the oldest globular
clusters will be more metal-poor compared to clusters formed recently.
