The Luminosity Function of OB Associations in the Galaxy
OB associations ionize the interstellar medium, producing both localized H II regions and diffuse ionized gas. The supernovae resulting from these associations pressurize and stir the interstellar medium. Using Smith, Biermann, & Mezger's compilation of radio H II regions in the Galaxy, and Kennicutt, Edgar, & Hodge's optical study of H II regions in nearby galaxies, we show that the luminosity distribution of giant OB associations in the Galaxy can be fit by a truncated power law of the form a(>S)=au[(Su/S)-1], where S is the ionizing photon luminosity, a(>S) is the number of associations with a luminosity of at least S, and Su is the upper limit to the distribution. The coefficient au is the number of the most luminous associations, with a luminosity between 0.5Su and Su. For the Galaxy, au=6.1; the fact that the number of the most luminous associations is significantly larger than unity indicates that there is a physical limit to the maximum size of H II regions in the Galaxy. To extend the luminosity distribution to small H II regions, we assume that the birthrate of associations, a(>*), is also a truncated power law, a(>*)∝[(*u/*)-1], where * is the number of stars in the association. For large associations, the ionizing luminosity is proportional to the number of stars, S∝*; for smaller associations, we use both an analytic and a Monte Carlo approach to find the resulting luminosity distribution a(>S).