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Astron. Astrophys. 330, 443-446 (1998)

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4. Discussion

In nearby starbursts, the photodissociated gas represents a substantial fraction (40 per cent) of the total gas mass in the nuclei, and the line-intensity ratio of [C II ] to CO(1-0) is 4100, with a very small scatter (Stacey et al. 1991). In the Galactic plane, the ratio is around 1300 (Nakagawa et al. 1993), whilst in low-metallicity regions such as 30 Dor, or the Large Magellanic Cloud (LMC) in general, the ratio is high (77000 and 23000 for 30 Dor and the LMC, respectively) because there are few dust grains to shield the molecular gas. The UV therefore penetrates deep inside each clump, dissociating CO and creating a thick skin of C [FORMULA] ions (Mochizuki et al. 1994). Note that although the metallicity is thought to have the dominant influence on the [C II ]/CO(1-0) line intensity ratio, the global UV field strength is also expected to have some effect.

Given that DLAAS at [FORMULA] are expected to be low-metallicity systems, perhaps similar in many respects to the LMC, the data we have presented (in particular the low limit on the [C II ]/CO(1-0) intensity ratio, based on the claimed CO(1-0) detection) support the view that the CO detections of Frayer et al. (1994) were spurious. There is, however, one proviso concerning the beams used to sample the emission region: the area of our beam (11[FORMULA] FWHM) was [FORMULA] times smaller than those used by Frayer et al., and if the emission region proves to be an order of magnitude larger than the Milky Way then we would not only have missed the majority of the emitting gas, but we may well have been chopping onto some of it.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998