  The formation of massive stars and their arrival on the zero-age
main-sequence occurs hidden behind dense clouds of gas and dust. In the giant
Hii region NGC 3603, the radiation of a young cluster of OB stars has dispersed
dust and gas in its vicinity. At a projected distance of 2:5 pc from the
cluster, a bright mid-infrared (mid-IR) source (IRS 9A) had been identified as
a massive young stellar object (MYSO), located on the side of a molecular clump
(MM2) of gas facing the cluster. We investigated the physical conditions in
MM2, based on APEX sub-mm observations using the SABOCA and SHFI instruments,
and archival ATCA 3 mm continuum and CS spectral line data. We resolved MM2
into several compact cores, one of them closely associated with IRS 9A. These
are likely infrared dark clouds as they do not show the typical hot-core
emission lines and are mostly opaque against the mid-IR background. The compact
cores have masses of up to several hundred times the solar mass and gas
temperatures of about 50 K, without evidence of internal ionizing sources. We
speculate that IRS 9A is younger than the cluster stars, but is in an
evolutionary state after that of the compact cores.
