Presentation #130.04 in the session Star Associations.
A long standing problem when deriving the physical properties of stellar populations is the degeneracy between age and reddening. When coupled with the assumption of the same metallicity for all star clusters in a galaxy, this combination can cause significant systematic errors when performing SED-fitting of star clusters in spiral galaxies, i.e., severely underestimating the ages and overestimating the reddening for old globular clusters, which typically have lower metallicity than recently formed clusters. As many as 20 % of all star clusters in a galaxy can have “catastrophic” errors in age a factor of ten or more, with a few percent of the old globular clusters being given ages less than 10 Myr. In this talk we describe a relatively simple way to fix the problem for old globular clusters, by first identifying candidate globular clusters using their colors, and then reassigning ages and reddening based on more appropriate lower metallicity simple stellar population models (i.e., 1/50th solar). The procedure is currently being tested on the PHANGS-HST (Physics at High Angular Resolution in Nearby Galaxies) sample of galaxies. Maximum reddening values of E(B-V) = 0.1 mag are employed for this population to force the solution to use lower metallicity models rather than searching for a higher metallicity solution with a higher value of reddening. Interlopers (i.e., young clusters with extensive dust that gives them colors similar to old globular clusters) are identified using either dust maps, Hα flux, or CO intensities, with Hα being the most effective. This procedure allows us to reduce the fraction of catastrophic errors from about 20 % to 5 % in most of the PHANGS-HST galaxies.