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We present a study of the effective (half-light) radii and other structuralproperties of a systematically selected sample of young, massive star clusters(YMCs, $\geq$$5\times10^3$ M$_{\odot}$ and $\leq$200 Myr) in two nearby spiralgalaxies, NGC 628 and NGC 1313. We use Hubble Space Telescope WFC3/UVIS andarchival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS),an HST Treasury Program. We measure effective radii with GALFIT, atwo-dimensional image-fitting package, and with a new technique to estimateeffective radii from the concentration index (CI) of observed clusters. Thedistribution of effective radii from both techniques spans $\sim$0.5-10 pc andpeaks at 2-3 pc for both galaxies. We find slight positive correlations betweeneffective radius and cluster age in both galaxies, but no significantrelationship between effective radius and galactocentric distance. Clusters inNGC 1313 display a mild increase in effective radius with cluster mass, but thetrend disappears when the sample is divided into age bins. We show that thevast majority of the clusters in both galaxies are much older than theirdynamical times, suggesting they are gravitationally bound objects. We findthat about half of the clusters in NGC 628 are underfilling their Roche lobes,based on their Jacobi radii. Our results suggest that the young, massiveclusters in NGC 628 and NGC 1313 are expanding due to stellar mass loss ortwo-body relaxation and are not significantly influenced by the tidal fields oftheir host galaxies.

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies