Star-forming Cloud Complexes in the Central Molecular Zone of NGC 253
  We report 350 and 230 GHz observations of molecular gas and dust in the
starburst nucleus of NGC 253 at 20-40 pc (1"-2") resolution. The data contain
CO(3-2), HCN(4-3), CO(2-1), 13CO(2-1), C18O(2-1), and continuum at 0.87 mm and
1.3 mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central
molecular zone (CMZ) is measured to be about 300 pc x 100 pc at the half
maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at
arcsecond resolution, and they are associated with compact radio sources due to
recent massive star formation. They contribute one third of the CO emission in
the central 300 pc and have 12CO peak brightness temperatures around 50 K,
molecular gas column densities on the order of 10^{4} Msun pc^{-2}, gas masses
on the order of 10^{7} Msun in the size scale of 20 pc, volume-averaged gas
densities of n(H2) ~ 4000 cm^{-3}, and high HCN-to-CO ratios suggestive of
higher fractions of dense gas than in the surrounding environment. It is
suggested that these are natal molecular cloud complexes of massive star
formation. The CMZ of NGC 253 is also compared with that of our Galaxy in
CO(2-1) at the same 20 pc resolution. Their overall gas distributions are
strikingly similar. The five molecular cloud complexes appear to be akin to
such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l=1.3deg cloud in the
Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher
temperatures and higher surface (and presumably volume) densities than its
non-starburst cousin.
