We report 350 and 230 GHz observations of molecular gas and dust in the starburst nucleus of ( catalog NGC 253 ) at 20–40 pc ( 1 ^ { \prime \prime } –2 ^ { \prime \prime } ) resolution .
The data contain CO ( 3–2 ) , HCN ( 4–3 ) , CO ( 2–1 ) , ^ { 13 } CO ( 2–1 ) , C ^ { 18 } O ( 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 \times 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 ^ { 12 } CO peak brightness temperatures around 50 K , molecular gas column densities on the order of 10 ^ { 4 } M _ { \odot } pc ^ { -2 } , gas masses on the order of 10 ^ { 7 } M _ { \odot } in the size scale of 20 pc , volume-averaged gas densities of \mbox { $n _ { H _ { 2 } } $ } \sim 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 = 1.3 \arcdeg 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 .