2009ApJ...706.1323M

UV radiation from massive stars is thought to be the dominant heating mechanism of the nuclear interstellar medium (ISM) in the late stages of evolution of starburst galaxies, creating large photodissociation regions (PDRs) and driving a very specific chemistry. We report the first detection of PDR molecular tracers, namely HOC⁺ and CO⁺, and also confirm the detection of the PDR tracer HCO toward the starburst galaxy NGC 253, claimed to be mainly dominated by shock heating and in an earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our CO⁺ detection suffers from significant blending to a group of transitions of ¹³CH₃OH, tentatively detected for the first time in the extragalactic ISM. These species are efficiently formed in the highly UV-irradiated outer layers of molecular clouds, as observed in the late stage nuclear starburst in M 82. The molecular abundance ratios we derive for these molecules are very similar to those found in M 82. This strongly supports the idea that these molecules are tracing the PDR component associated with the starburst in the nuclear region of NGC 253. The presence of large abundances of PDR molecules in the ISM of NGC 253, which is dominated by shock chemistry, clearly illustrates the potential of chemical complexity studies to establish the evolutionary state of starbursts in galaxies. A comparison with the predictions of chemical models for PDRs shows that the observed molecular ratios are tracing the outer layers of UV-illuminated clouds up to two magnitudes of visual extinction. We combine the column densities of PDR tracers reported in this paper with those of easily photodissociated species, such as HNCO, to derive the fraction of material in the well-shielded core relative to the UV-pervaded envelopes. Chemical models, which include grain formation and photodissociation of HNCO, support the scenario of a photo-dominated chemistry as an explanation to the abundances of the observed species. From this comparison, we conclude that the molecular clouds in NGC 253 are more massive and with larger column densities than those in M 82, as expected from the evolutionary stage of the starbursts in both galaxies.