Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM


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

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Observations of the galaxy WLM.


  1. Krumholz, M. R. Star formation in atomic gas. Astrophys. J. 759, 9 (2012)

    Article  ADS  Google Scholar 

  2. 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)

    CAS  Article  ADS  Google Scholar 

  3. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009)

    CAS  Article  ADS  Google Scholar 

  4. 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)

    Article  ADS  Google Scholar 

  5. Taylor, C. L. & Klein, U. A search for CO in the Local Group dwarf irregular galaxy WLM. Astron. Astrophys. 366, 811–816 (2001)

    CAS  Article  ADS  Google Scholar 

  6. Bigiel, F. et al. A constant molecular gas depletion time in nearby disk galaxies. Astrophys. J. 730, L13 (2011)

    Article  ADS  Google Scholar 

  7. Taylor, C. L., Kobulnicky, H. A. & Skillman, E. D. CO emission in low-luminosity, H I-rich galaxies. Astron. J. 116, 2746–2756 (1998)

    CAS  Article  ADS  Google Scholar 

  8. Leroy, A. K. et al. The CO-to-H2 conversion factor from infrared dust emission across the Local Group. Astrophys. J. 737, 12 (2011)

    Article  ADS  Google Scholar 

  9. Schruba, A. et al. Low CO luminosities in dwarf galaxies. Astron. J. 143, 138 (2012)

    Article  ADS  Google Scholar 

  10. Vassilev, V. et al. A Swedish heterodyne facility instrument for the APEX telescope. Astron. Astrophys. 490, 1157–1163 (2008)

    Article  ADS  Google Scholar 

  11. Siringo, G. et al. The Large APEX BOlometer CAmera LABOCA. Astron. Astrophys. 497, 945–962 (2009)

    Article  ADS  Google Scholar 

  12. Dale, D. A. et al. The Spitzer Local Volume Legacy: survey description and infrared photometry. Astrophys. J. 703, 517–556 (2009)

    CAS  Article  ADS  Google Scholar 

  13. Planck Collaboration et al. Planck early results. XXV. Thermal dust in nearby molecular clouds. Astron. Astrophys. 536, A25 (2011)

  14. Draine, B. T. Interstellar dust grains. Annu. Rev. Astron. Astrophys. 41, 241–289 (2003)

    Article  ADS  Google Scholar 

  15. 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)

    Article  ADS  Google Scholar 

  16. Coupeaud et al. Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues. Astron. Astrophys. 535, A124 (2011)

    Article  Google Scholar 

  17. 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)

    Article  Google Scholar 

  18. Verdugo, C. Sub-Millimeter Studies of Cold Dust and Gas in the Magellanic Clouds. MSc thesis, Univ. Chile. (2012)

  19. Planck Collaboration. Planck early results. XVII. Origin of the submillimetre excess dust emission in the Magellanic Clouds. Astron. Astrophys. 536, A17 (2011)

  20. Draine, B. T. et al. Dust masses, PAH abundances, and starlight intensities in the SINGS galaxy sample. Astrophys. J. 663, 866–894 (2007)

    CAS  Article  ADS  Google Scholar 

  21. 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)

    Article  ADS  Google Scholar 

  22. Dufour, R. J. The composition of H II regions in the Magellanic Clouds. IAU Symp. 108, 353–360 (1984)

    ADS  Google Scholar 

  23. 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)

    CAS  Article  ADS  Google Scholar 

  24. Leroy, A. K. et al. The star formation efficiency in nearby galaxies: measuring where gas forms stars effectively. Astron. J. 136, 2782–2845 (2008)

    CAS  Article  ADS  Google Scholar 

  25. 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)

    Article  ADS  Google Scholar 

  26. 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)

    CAS  Article  ADS  Google Scholar 

  27. Engelbracht, C. W. et al. Metallicity effects on dust properties in starbursting galaxies. Astrophys. J. 678, 804–827 (2008)

    CAS  Article  ADS  Google Scholar 

  28. 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)

    Article  ADS  Google Scholar 

  29. McMillan, P. J. Mass models of the Milky Way. Mon. Not. R. Astron. Soc. 414, 2446–2457 (2011)

    Article  ADS  Google Scholar 

  30. 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)

    Article  ADS  Google Scholar 

Download references


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.

Author information

Authors and Affiliations



B.G.E. coordinated the observational team, did the calculations for Table 2 and wrote the manuscript; M.R. was principal investigator for Chilean observing time on the APEX telescope and, with C.V., observed the galaxy in CO and at 870 μm, reduced the relevant data in Table 1 and did relevant calculations for Table 2; D.A.H. determined the observational strategy, selected WLM for study, chose the observing coordinates, extracted the H i spectra from the LITTLE THINGS data and prepared Fig. 1. E.B. was principal investigator on the APEX proposal using European time through ESO and coordinated the work on data uncertainties and background noise. A.S. made the WLM H i data cube from Jansky Very Large Array observations. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Bruce G. Elmegreen.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Tables

This file contains Supplementary Table 3 (see main text for Tables 1 and 2). (PDF 44 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

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).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing