Warm molecular gas in dwarf starburst galaxies: CO(3-2) observations

Eight dwarf starburst galaxies have been observed with the Caltech Submillimeter Observatory telescope in the CO J = 3-2 transition. The galaxies observed are He 2-10, NGC 5253, 1569, and 3077, Haro 2, Haro 3, II Zw 40, and Mrk 86; all but the last two are detected. The central regions of He 2-10 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 is 0.60 +/- 0.06, which is virtually identical to that found for starbursts in the nuclei of nearby spirals and suggests that the molecular gas is optically thick, warm (T-k > 20 K), and moderately dense (n(H2) similar to 10(3)-10(4) cm(-3)). The CO(3-2)/CO(1-0) ratio peaks at or close to the starburst in all cases. CO emission does not appear to be optically thin in these dwarfs, despite the low metallicity and intense radiation fields, probably because for CO to exist in detectable amounts it must be self-shielding and hence optically thick. Physical properties of the molecular clouds in these dwarf starbursts appear to be essentially the same as those in nearby spiral nuclei, with the possible exception that CO is more confined to the cloud cores.