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), (CO)-C-13(2-1), (CO)-O-18(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 (CO)-C-12 peak brightness temperatures around 50 K, molecular gas column densities on the order of 10(4) M-circle dot pc(-2), gas masses on the order of 10(7) M-circle dot in the size scale of 20 pc, volume-averaged gas densities of n(H2) similar to 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 degrees.3 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.