OB ASSOCIATIONS AT THE UPPER END OF THE MILKY WAY LUMINOSITY FUNCTION

The Milky Way's most luminous, young, and massive (M ∼> 104 M☉) star clusters and OB associations have largely evaded detection despite knowledge of their surrounding H II regions. We search for these clusters and associations within the 40 star-forming complexes from Rahman and Murray in the 13 most luminous Wilkinson Microwave Anisotropy Probe (WMAP) free-free emission sources of the Galaxy. Selecting for objects with the dust-reddened colors of OB stars, we identify new candidate associations using the Two Micron All Sky Survey point-source catalog. In 40 star-forming complexes searched, 22 contain cluster/association candidates with sizes and masses in the range of 3'-26' and 102.3-105 M☉. Of the 22 candidates, at least seven have estimated masses ∼> 104 M☉, doubling the number of such massive clusters known in the Galaxy. Only one of the searched WMAP sources remains without a candidate. Applying our method to a statistically similar set of test locations, we estimate that 3.0 ± 0.6 of our 22 candidate associations are unrelated to the star-forming complexes. In addition, the apparent extinctions of our candidate associations correlate well with the predictions from a Galactic model. These facts, along with the clear detection of a known OB association and the previous spectral verification of one cluster found by this method, validate our method. In eight of the most luminous WMAP sources, the candidate associations can account for the observed free-free flux. With our new compilation, the Galactic census of young, massive stellar associations may now be about half complete.