Journal Article

Minimal HCN emission from molecular clouds in M33

Erik Rosolowsky, Jaime E. Pineda and Yu Gao

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 2, pages 1977-1984
Published in print August 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI:
Minimal HCN emission from molecular clouds in M33

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Since HCN emission has been shown to be a linear tracer of ongoing star formation activity, we have searched for HCN (J= 1 → 0) emission from known GMCs in the nearby galaxy M33. No significant HCN emission has been found along any of the lines of sight. We find two lines of sight with CO-to-HCN integrated intensity ratios up to 280, nearly a factor of 6 above what is found in comparable regions of the Milky Way. Star formation tracers suggest that the HCN-to-star formation rate ratio () is a factor of 6 lower than what is observed in the Milky Way (on average) and local extragalactic systems. Simple chemical models accounting for the subsolar N/O ratio suggest that depletion cannot account for the high CO-to-HCN ratios. Given HCN formation requires high extinction (AV > 4), low metallicity may yield reduced dust shielding and thus a high CO/HCN ratio. The turbulence and structure of GMCs in M33 are comparable to those found in other systems, so the differences are unlikely to result from different GMC properties. Since lower CO-to-HCN ratios are associated with the highest rates of star formation, we attribute the deficits in part to evolutionary effects within GMCs.

Keywords: stars: formation; ISM: clouds; galaxies: individual: M33; radio lines: ISM

Journal Article.  5650 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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