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We present a new photometric catalog of 326 candidate globular clusters (GCs)in the nearby spiral galaxy M101, selected from B, V, and I Hubble SpaceTelescope Advanced Camera for Surveys images. The luminosity function (LF) ofthese clusters has an unusually large number of faint sources compared withGCLFs in many other spiral galaxies. Accordingly, we separate and compare theproperties of "bright" (M_V < -6.5) versus "faint" (M_V > -6.5; one magnitudefainter than the expected GC peak) clusters within our sample. The LF of thebright clusters is well fit by a peaked distribution similar to those observedin the Milky Way (MW) and other galaxies. These bright clusters also havesimilar size (r_{eff}) and spatial distributions as MW GCs. The LF of the faintclusters, on the other hand, is well described by a power law, dN(L_V)/dL_Vproportional to L_V^alpha with alpha = -2.6 plus or minus 0.3, similar to thoseobserved for young and intermediate-age cluster systems in star forminggalaxies. We find that the faint clusters have larger typical r_{eff} than thebright clusters, and have a flatter surface density profile, being more evenlydistributed, as we would expect for clusters associated with the disk. We usethe shape of the LF and predictions for mass-loss driven by two-body relaxationto constrain the ages of the faint clusters. Our results are consistent withtwo populations of old star clusters in M101: a bright population of haloclusters and a fainter, possibly younger, population of old disk clusters.

Two Populations of Old Star Clusters in the Spiral Galaxy M101 Based on HST/ACS Observations