Age and Mass Signatures in the Multiband Second-order Image Structure of Young Star Clusters in M31

Abstract

The ages and masses of star clusters may leave systematic signatures in the second-order spatial structure of semi-resolved photometric images. To test this hypothesis, we analyze the images using the Hessian matrix and identify basic luminous structures, termed peak cores. Applying this analysis to six-band PHAT/AP images of 1249 M31 clusters with CMD-based ages of 10--300 Myr and masses of $10^{2.2}$--$10^{4.5}\,M_{\odot}$, we examine the correlations of age and mass with the peak-core structural parameters, $C_S$ and $Q$. Younger clusters show larger $C_S$ values in the ultraviolet bands, with the strongest age--$C_S$ anti-correlation in F336W ($ρ=-0.61$), whereas more massive clusters show smaller $Q$ values, most clearly in F336W ($ρ=-0.44$). These correlations suggest that the contrast of UV-bright peak cores decreases with age, whereas increasing mass alters the abundance and spatial organization of detectable structures across different bands.