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

Eight dwarf starburst galaxies have been observed with the Caltech Submilli meter Observatory telescope in the CO J = 3-2 transition. The galaxies obse rved are He 2-10, NGC 5253, 1569, and 3077, Haro 2, Haro 3, II Zw 40, and M rk 86; all but the last two are detected. The central regions of He 2-10 an d 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 becaus e for CO to exist in detectable amounts it must be self-shielding and hence optically thick. Physical properties of the molecular clouds in these dwar f starbursts appear to be essentially the same as those in nearby spiral nu clei, with the possible exception that CO is more confined to the cloud cor es.