We use UBVI H \alpha images of the Whirlpool galaxy , M51 , taken with the ACS and WFPC2 cameras on the Hubble Space Telescope ( HST ) to select star clusters , and to estimate their masses and ages by comparing their observed colors with predictions from population synthesis models .
We construct the mass function of intermediate age ( 1– 4 \times 10 ^ { 8 } yr ) clusters , and find that it is well described by a power law , \psi ( M ) \propto M ^ { \beta } , with \beta = -2.1 \pm 0.2 , for clusters more massive than M \approx 6 \times 10 ^ { 3 } ~ { } M _ { \odot } .
This extends the mass function of intermediate age clusters in M51 to masses lower by nearly a factor of five over previous determinations .
The mass function does not show evidence for curvature at either the high or low mass end .
This shape indicates that there is no evidence for the earlier disruption of lower mass clusters compared with their higher mass counterparts ( i.e. , no mass-dependent disruption ) over the observed range of masses and ages , or for a physical upper mass limit M _ { C } with which clusters in M51 can form .
These conclusions differ from previous suggestions based on poorer-quality HST observations .
We discuss their implications for the formation and disruption of the clusters .
Ages of clusters in two “ feathers , ” stellar features extending from the outer portion of a spiral arm , show that the feather with a larger pitch angle formed earlier , and over a longer period , than the other .