Warm Molecular Gas in Dwarf Starburst Galaxies: CO(3–2) Observations

Eight dwarf starburst galaxies have been observed with the CaltechSubmillimeter Observatory (CSO) telescope in the CO J= 3 - 2 transition. Thegalaxies observed are He 2-10, NGC 5253, NGC 1569, NGC 3077, Haro 2, Haro 3, IIZw 40 and Mrk 86; all but the last two are detected. The central regions of He2-10 and NGC 5253 were mapped and a CO(2-1) spectrum of NGC 5253 was obtained.The error weighted mean CO(3-2)/CO(1-0) ratio of the detected galaxies is0.60$\pm$0.06, which is virtually identical to what is found for starbursts inthe nuclei of nearby spirals, and suggests that the molecular gas is opticallythick, warm (T$_{k}>$20 K), and moderately dense ($n_{H_{2}}\sim 10^{3-4}cm^{-3}$). The CO(3-2)/CO(1-0) ratio peaks at or close to the starburst in allcases. CO emission does not appear to be optically thin in these dwarfs,despite the low metallicity and intense radiation fields, which is probablybecause in order for CO to exist in detectable amounts it must beself-shielding and hence optically thick. Physical properties of the molecularclouds in these dwarf starbursts appear to be essentially the same as nearbyspiral nuclei, with the possible exception that CO is more confined to thecloud cores.Comment: 21 pages, 8 figures; Accepted for publication by the Astronomical Journa