Photodissociation chemistry footprints in the Starburst galaxy NGC 253

TL;DR

This study detects and analyzes PDR molecular tracers in NGC 253, revealing that its molecular clouds are more massive and UV-illuminated than in M 82, supporting a photo-dominated chemistry scenario.

Contribution

First detection of PDR tracers HOC+, CO+, and confirmation of HCO in NGC 253, linking molecular abundances to PDR chemistry in an extragalactic starburst.

Findings

Molecular abundance ratios similar to M 82, indicating PDR dominance.

Molecular clouds in NGC 253 are more massive with larger column densities.

Chemical models support UV-driven chemistry in the observed regions.

Abstract

We report the first detection of PDR molecular tracers, namely HOC+, and CO+, and confirm the detection of the also PDR tracer HCO towards 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 interstellar medium. 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. 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. 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.