NIR Subarcsec Images of W49A: Uncovering the Beast

Joao Alves, Nicole Homeier

We present subarcsecond J H and Ks images (FWHM ˜

0.5'') of an unbiased 5'×5' (16 pc × 16 pc) survey of the

densest region of the W49 giant molecular cloud. The observations

reveal 4 massive stellar clusters (with stars as massive as ˜ 120

M⊙) the larger (Cluster 1) about 3 pc East of the well

known Welch ring of ultra-compact HII regions. Cluster 1 is a)

extincted by at least AV > 20 mag of foreground (unrelated

and local) extinction b) has more than 30 magnitudes of internal

inhomogeneous extinction implying that it is still deeply buried in its

parental molecular cloud and c) is powering a 6 pc diameter giant HII

region seen both at the NIR and radio continuum. We also identify the

exciting sources of several UC HII 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

multi-seeded instead of resulting from a coherent trigger

Department of Astrophysics
External organisation(s)
Joint ALMA Observatory
Publication date
Portal url