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 Space Telescope Advanced Camera for Surveys images .
The luminosity function ( LF ) of these clusters has an unusually large number of faint sources compared with GCLFs in many other spiral galaxies .
Accordingly , we separate and compare the properties of “ bright ” ( M _ { V } < -6.5 ) versus “ faint ” ( M _ { V } > -6.5 ; one magnitude fainter than the expected GC peak ) clusters within our sample .
The LF of the bright clusters is well fit by a peaked distribution similar to those observed in the Milky Way ( MW ) and other galaxies .
These bright clusters also have similar size ( r _ { \text { eff } } ) and spatial distributions as MW GCs .
The LF of the faint clusters , on the other hand , is well described by a power law , dN ( L _ { V } ) / dL _ { V } \propto L _ { V } ^ { \alpha } with \alpha = -2.6 \pm 0.3 , similar to those observed for young and intermediate-age cluster systems in star forming galaxies .
We find that the faint clusters have larger typical r _ { \text { eff } } than the bright clusters , and have a flatter surface density profile , being more evenly distributed , as we would expect for clusters associated with the disk .
We use the shape of the LF and predictions for mass-loss driven by two-body relaxation to constrain the ages of the faint clusters .
Our results are consistent with two populations of old star clusters in M101 : a bright population of halo clusters and a fainter , possibly younger , population of old disk clusters .