2011ApJ...735...75L

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. 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. 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 ∼10³ 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 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.