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.
This is a preview of subscription content, access via your institution
Open Access articles citing this article.
Experimental Astronomy Open Access 01 June 2021
Nature Communications Open Access 09 December 2016
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
Krumholz, M. R. Star formation in atomic gas. Astrophys. J. 759, 9 (2012)
Lee, H., Skillman, E. D. & Venn, K. A. Investigating the possible anomaly between nebular and stellar oxygen abundances in the dwarf irregular galaxy WLM. Astrophys. J. 620, 223–237 (2005)
Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009)
Leaman, R. et al. The resolved structure and dynamics of an isolated dwarf galaxy: a VLT and Keck spectroscopic survey of WLM. Astrophys. J. 750, 33 (2012)
Taylor, C. L. & Klein, U. A search for CO in the Local Group dwarf irregular galaxy WLM. Astron. Astrophys. 366, 811–816 (2001)
Bigiel, F. et al. A constant molecular gas depletion time in nearby disk galaxies. Astrophys. J. 730, L13 (2011)
Taylor, C. L., Kobulnicky, H. A. & Skillman, E. D. CO emission in low-luminosity, H I-rich galaxies. Astron. J. 116, 2746–2756 (1998)
Leroy, A. K. et al. The CO-to-H2 conversion factor from infrared dust emission across the Local Group. Astrophys. J. 737, 12 (2011)
Schruba, A. et al. Low CO luminosities in dwarf galaxies. Astron. J. 143, 138 (2012)
Vassilev, V. et al. A Swedish heterodyne facility instrument for the APEX telescope. Astron. Astrophys. 490, 1157–1163 (2008)
Siringo, G. et al. The Large APEX BOlometer CAmera LABOCA. Astron. Astrophys. 497, 945–962 (2009)
Dale, D. A. et al. The Spitzer Local Volume Legacy: survey description and infrared photometry. Astrophys. J. 703, 517–556 (2009)
Planck Collaboration et al. Planck early results. XXV. Thermal dust in nearby molecular clouds. Astron. Astrophys. 536, A25 (2011)
Draine, B. T. Interstellar dust grains. Annu. Rev. Astron. Astrophys. 41, 241–289 (2003)
Galametz, M. et al. Mapping the cold dust temperatures and masses of nearby KINGFISH galaxies with Herschel. Mon. Not. R. Astron. Soc. 425, 763–787 (2012)
Coupeaud et al. Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues. Astron. Astrophys. 535, A124 (2011)
Galametz, M. et al. Probing the dust properties of galaxies up to submillimetre wavelengths. II. Dust-to-gas mass ratio trends with metallicity and the submm excess in dwarf galaxies. Astron. Astrophys. 532, A56 (2011)
Verdugo, C. Sub-Millimeter Studies of Cold Dust and Gas in the Magellanic Clouds. MSc thesis, Univ. Chile. (2012)
Planck Collaboration. Planck early results. XVII. Origin of the submillimetre excess dust emission in the Magellanic Clouds. Astron. Astrophys. 536, A17 (2011)
Draine, B. T. et al. Dust masses, PAH abundances, and starlight intensities in the SINGS galaxy sample. Astrophys. J. 663, 866–894 (2007)
Nikolić, S., Garay, G., Rubio, M. & Johansson, L. E. B. CO and CS in the Magellanic Clouds: a χ2-analysis of multitransitional data based on the MEP radiative transfer model. Astron. Astrophys. 471, 561–571 (2007)
Dufour, R. J. The composition of H II regions in the Magellanic Clouds. IAU Symp. 108, 353–360 (1984)
Hunter, D. A., Elmegreen, B. G. & Ludka, B. C. GALEX ultraviolet imaging of dwarf galaxies and star formation rates. Astron. J. 139, 447–475 (2010)
Leroy, A. K. et al. The star formation efficiency in nearby galaxies: measuring where gas forms stars effectively. Astron. J. 136, 2782–2845 (2008)
Lada, C. J., Forbrich, J., Lombardi, M. & Alves, J. F. Star formation rates in molecular clouds and the nature of the extragalactic scaling relations. Astrophys. J. 745, 190 (2012)
Mannucci, F. et al. LSD: Lyman-break galaxies, stellar populations and dynamics – I. Mass, metallicity and gas at z ∼ 3.1. Mon. Not. R. Astron. Soc. 398, 1915–1931 (2009)
Engelbracht, C. W. et al. Metallicity effects on dust properties in starbursting galaxies. Astrophys. J. 678, 804–827 (2008)
Zhang, H.-X., Hunter, D. A., Elmegreen, B. G., Gao, Y. & Schruba, A. Outside-in shrinking of the star-forming disks of dwarf irregular galaxies. Astron. J. 143, 47 (2012)
McMillan, P. J. Mass models of the Milky Way. Mon. Not. R. Astron. Soc. 414, 2446–2457 (2011)
Chomiuk, L. & Povich, M. S. Toward a unification of star formation rate determinations in the Milky Way and other galaxies. Astron. J. 142, 197 (2011)
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.
About this article
Cite this article
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
Experimental Astronomy (2021)
Nature Communications (2016)
The Astronomy and Astrophysics Review (2016)