Carbon monoxide (CO) is the primary tracer for interstellar clouds where stars form, but it has never been detected in galaxies in which the oxygen abundance relative to hydrogen is less than 20 per cent of that of the Sun, even though such ‘low-metallicity’ galaxies often form stars. This raises the question of whether stars can form in dense gas without molecules, cooling to the required near-zero temperatures by atomic transitions and dust radiation rather than by molecular line emission1; and it highlights uncertainties about star formation in the early Universe, when the metallicity was generally low. Here we report the detection of CO in two regions of a local dwarf irregular galaxy, WLM, where the metallicity is 13 per cent of the solar value2,3. We use new submillimetre observations and archival far-infrared observations to estimate the cloud masses, which are both slightly greater than 100,000 solar masses. The clouds have produced stars at a rate per molecule equal to 10 per cent of that in the local Orion nebula cloud. The CO fraction of the molecular gas is also low, about 3 per cent of the Milky Way value. These results suggest that in small galaxies both star-forming cores and CO molecules become increasingly rare in molecular hydrogen clouds as the metallicity decreases.
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This work was funded in part by the US National Science Foundation through grants AST-0707563 and AST-0707426 to D.A.H. and B.G.E. M.R. and C.V. wish to acknowledge support from CONICYT (FONDECYT grant no. 1080335). M.R. was also supported by the Chilean Center for Astrophysics FONDAP grant no. 15010003. A.S. was supported by the Deutsche Forschungsgemeinschaft Priority Program 1177. We are grateful to M. Albrecht for help with the LABOCA data reduction and to L. Hill for making Fig. 1a. The National Radio Astronomy Observatory is a facility of the US National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
The authors declare no competing financial interests.
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Elmegreen, B., Rubio, M., Hunter, D. et al. Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM. Nature 495, 487–489 (2013). https://doi.org/10.1038/nature11933
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