Uncovering the beast: Discovery of embedded massive stellar clusters in W49A

We present subarcsecond J, H, and K-s images (FWHM similar to 0."5) of an unbiased 5' x 5' (16 x 16 pc) survey of the densest region of the W49 giant molecular cloud. The observations reveal four massive stellar clusters ( with stars as massive as similar to120 M-circle dot), the largest (cluster 1) about 3 pc east of the well-known Welch ring of ultracompact, H II regions. Cluster 1 is (i) extincted by at least A(V) > 20 mag of foreground (unrelated and local) extinction, (ii) has more than 30 mag of internal inhomogeneous extinction, implying that it is still deeply buried in its parental molecular cloud, and (iii) is powering a 6 pc diameter giant H ii region seen at both the near-infrared and the radio continuum. We also identify the exciting sources of several ultracompact H II regions. The census of massive stars in W49A agrees or is slightly overabundant when compared with the number of Lyman continuum photons derived from radio observations. We argue that although the formation of the Welch ring could have been triggered by cluster 1, the entire W49A starburst region seems to have been multiseeded instead of resulting from a coherent trigger.