The H II region luminosity function of M51

A study is presented of the H II region luminosity function (LF) of M51, complete down to a luminosity of about L = 10³7. 6^ ergs s^-1^. A total of 616 H II regions have been identified from a deep Hα exposure: 480 in the "arm" regions, and 136 in the "interarm" regions. The differential LF shows a significant steepening above L = 10³9. 0^ ergs s^-1^. In the region below this break but above the completeness limit, where almost all of the H II regions lie, the slope of the LF is - 1. 55, which is rather shallow compared to other spirals and even irregular galaxies. The 616 H II regions account for about 45% of the total Hα luminosity of 1. 8 X 10⁴1^ ergs s^-1^. Undetected, low-luminosity H II regions probably account for only a small fraction of the remaining luminosity, so there is most likely a significant diffuse ionized medium in M51. A real difference in slope is found between the arm and interarm LFs over the luminosity range extending from the completeness limits to the break at 10³9. 0^ ergs s^-1^: - 1. 48+/-0. 07 vs - 2. 05+/0. 15. All of the H II regions above the break are in the arms. The arm H II regions account for more than ten times as much luminosity as the interarm H II regions, confirming the very high degree of concentration of massive star formation to the arms. Since the contrast in molecular gas is only about four to one, the enhanced efficiency of massive star formation in the arms reported by Vogel et al. Nature, 334, 402 (1988) is therefore confirmed. The slope of the LF and its spatial variations are interpreted in terms of the star formation process in M51, and are related to our understanding of the molecular gas distribution. The effects of blending of H II regions on the LF are addressed in an Appendix.