H Ii Region Luminosity Function of the Interacting Galaxy M51

We present a study of H ii regions in M51 using the Hubble Space Telescope Advanced Camera for Surveys images taken as part of the Hubble Heritage Program. We have cataloged about 19,600 H ii regions in M51 with Hα luminosity in the range of L = 10 35.5 –10 39.0 erg s −1 . The Hα luminosity function of H ii regions (H ii LF) in M51 is well represented by a double power law with its index α = −2.25 ± 0.02 for the bright part and α = −1.42 ± 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 ∼10 3 M ☉ , 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: H ii regions in the interarm region are relatively older than those in the other parts of M51.