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We present a ~1" (100 pc) resolution 12CO (3-2) map of the nearby intermediate-stage interacting galaxy pair NGC 4038/9 (the Antennae galaxies) obtained with the Submillimeter Array. We find that half the CO (3-2) emission originates in the overlap region where most of the tidally induced star formation had been previously found in shorter wavelength images, with the rest being centered on each of the nuclei. The gross distribution is consistent with lower resolution single-dish images, but we show for the first time the detailed distribution of the warm and dense molecular gas across this galaxy pair at resolutions comparable to the size of a typical giant molecular complex. While we find that 58% (33/57) of the spatially resolved Giant Molecular Associations (a few x 100 pc) are located in the overlap region, only ≤ 30% spatially coincides with the optically detected star clusters, suggesting that the bulk of the CO (3-2) emission traces the regions with very recent or near future star formation activity. The spatial distribution of the CO (3-2)/CO (1-0) integrated brightness temperature ratios mainly ranges between 0.3 and 0.6, which suggests that on average the CO (3-2) line in the Antennae is not completely thermalized and similar to the average values of nearby spirals. A higher ratio is seen in both nuclei and the southern complexes in the overlap region. A higher radiation field associated with intense star formation can account tbr the nucleus of NGC 4038 and the overlap region, but the nuclear region of NGC 4039 shows relatively little star formation or active galactic nucleus activity and cannot be easily explained. We show kinematical evidence that the high line ratio in NGC 4039 is possibly caused by gas inflow into the counter-rotating central disk. [ABSTRACT FROM AUTHOR]

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 × 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 104 M... pc-2, gas masses on the order of 107 M... in the size scale of 20 pc, volume-averaged gas densities of nH2 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 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. [ABSTRACT FROM AUTHOR]

We have used high-resolution (∼2.″3) observations of the local (DL = 46 Mpc) luminous infrared galaxy Arp 299 to map out the physical properties of the molecular gas that provides the fuel for its extreme star formation activity. The 12CO J = 3-2, 12CO J = 2-1, and 13CO J = 2-1 lines were observed with the Submillimeter Array, and the short spacings of the 12CO J = 2-1 and J = 3-2 observations have been recovered using the James Clerk Maxwell Telescope single dish observations. We use the radiative transfer code RADEX to estimate the physical properties (density, column density, and temperature) of the different regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694 and NGC 3690, are consistent with a wide range of gas components, from warm moderately dense gas with Tkin > 30 K and n(H2) ∼ 0.3-3 × 103 cm–3 to cold dense gas with Tkin ∼ 10-30 K and n(H2) > 3 × 103 cm–3. The overlap region is shown to have a better constrained solution with Tkin ∼ 10-50 K and n(H2) ∼ 1-30 × 103 cm–3. We estimate the gas masses and star formation rates of each region in order to derive molecular gas depletion times. The depletion times of all regions (20-50 Myr) are found to be about two orders of magnitude lower than those of normal spiral galaxies. This rapid depletion time can probably be explained by a high fraction of dense gas on kiloparsec scales in Arp 299. We estimate the CO-to-H2 factor, αco to be 0.4 ± 0.3(3 × 10–4/xCO) M☼ (K km s–1 pc2)–1 for the overlap region. This value agrees well with values determined previously for more advanced merger systems. [ABSTRACT FROM AUTHOR]