Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations they are molecular, with H2 the dominant species and carbon monoxide (CO) the best available tracer1,2. When the abundances of carbon and oxygen are low compared with that of hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies3, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds4. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf–Lundmark–Melotte (WLM)5, which has a metallicity that is 13 per cent of the solar value6,7 and 50 per cent lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.
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We wish to thank P. Massey and the Local Group Survey team for the use of their Hα image of WLM. L. Hill made the colour composite inset in Fig. 1. M.R. would like to thank C. Herrera and J. Garcia for support with the CASA implementation to reduce the raw data and A. Rojas for support in the ALMA data reduction. M.R. is grateful to A. Leroy for providing the galaxy data to produce Fig. 3. M.R. thanks the ALMA Director for the invitation to spend her 2015 sabbatical leave at the Joint ALMA Observatory (JAO) in Santiago, where this article was finished. P.C. is grateful to L. Young and S. Madden for invaluable guidance on Herschel data reduction. M.R. wishes to acknowledge support from the Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) through FONDECYT grant no. 1140839. M.R. is partially supported by CONICYT project BASAL PFB-06. The contributions from D.A.H. were funded by the Lowell Observatory Research Fund. P.C. acknowledges support from NASA (National Aeronautics and Space Administration) JPL RSA grant 1433776 to L. Young and grant 1456896 to D.A.H. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.
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