We present a study of H ii regions in M51 using the Hubble Space Telescope ACS images taken as part of the Hubble Heritage Program .
We have catalogued about 19,600 H ii regions in M51 with H \alpha luminosity in the range of L = 10 ^ { 35.5 } erg s ^ { -1 } to 10 ^ { 39.0 } erg s ^ { -1 } .
The H \alpha luminosity function of H ii regions ( H ii LF ) in M51 is well represented by a double power law with its index \alpha = -2.25 \pm 0.02 for the bright part and \alpha = -1.42 \pm 0.01 for the faint part , separated at a break point L = 10 ^ { 37.1 } erg s ^ { -1 } .
This break was not found in previous studies of M51 H ii regions .
Comparison with simulated H ii LFs suggests that this break is caused by the transition of H ii region ionizing sources , from low mass clusters ( with \sim 10 ^ { 3 } M _ { \odot } , including several OB stars ) to more massive clusters ( including several tens of OB stars ) .
The H ii LFs with L < 10 ^ { 37.1 } erg s ^ { -1 } are found to have different slopes for different parts in M51 : the H ii LF for the interarm region is steeper than those for the arm and the nuclear regions .
This observed difference in H ii LFs can be explained by evolutionary effects that H ii regions in the interarm region are relatively older than those in the other parts of M51 .