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.

Massive star formation within the Leo ‘primordial’ ring


Few intergalactic, plausibly primordial clouds of neutral atomic hydrogen (H i) have been found in the local Universe, suggesting that such structures have either dispersed, become ionized or produced a stellar population on gigayear timescales. The Leo ring1,2, a massive (MH i ≈ 1.8 × 109, denoting the solar mass), 200-kpc-wide structure orbiting the galaxies M105 and NGC 3384 with a 4-Gyr period, is a candidate primordial cloud. Despite repeated atttempts3,4, it has previously been seen only from H i emission, suggesting the absence of a stellar population. Here we report the detection of ultraviolet light from gaseous substructures of the Leo ring, which we attribute to recent massive star formation. The ultraviolet colour of the detected complexes is blue, implying the onset of a burst of star formation or continuous star formation of moderate (108-yr) duration. Measured ultraviolet–visible photometry favours models with low metallicity (Z ≈ /50–/5, denoting the solar metallicity), that is, a low proportion of elements heavier than helium, although spectroscopic confirmation is needed. We speculate that the complexes are dwarf galaxies observed during their formation, but distinguished by their lack of a dark matter component5. In this regard, they resemble tidal dwarf galaxies, although without the enrichment preceding tidal stripping. If structures like the Leo ring were common in the early Universe, they may have produced a large, yet undetected, population of faint, metal-poor, halo-lacking dwarf galaxies.

This is a preview of subscription content

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: H  i structure of the Leo ring, compared to the distribution of galaxies in the M96 group.
Figure 2: Ultraviolet imaging compared with the H  i distribution.
Figure 3: Ultraviolet and visible imaging of detected stellar complexes.
Figure 4: Colour–colour diagram comparing the Leo ring regions with population synthesis models14.


  1. Schneider, S. E., Helou, G., Salpeter, E. E. & Terzian, Y. Discovery of a large intergalactic HI cloud in the M96 group. Astrophys. J. 273, L1–L15 (1983)

    ADS  CAS  Article  Google Scholar 

  2. Schneider, S. E. et al. Multifrequency survey of the intergalactic cloud in the M96 group. Astron. J. 97, 666–673 (1989)

    ADS  CAS  Article  Google Scholar 

  3. Pierce, M. J. & Tully, R. B. The extragalactic HI cloud in Leo. Astron. J. 90, 450–453 (1985)

    ADS  CAS  Article  Google Scholar 

  4. Kibblewhite, E. J., Cawson, M. G. M., Disney, M. J. & Phillips, S. An optical search for the intergalactic HI cloud in Leo. Mon. Not. R. Astron. Soc. 213, 111–115 (1985)

    ADS  CAS  Article  Google Scholar 

  5. Schneider, S. E., Salpeter, E. E. & Terzian, Y. High-resolution observations of the intergalactic HI cloud in the M96 group. Astron. J. 91, 13–22 (1986)

    ADS  CAS  Article  Google Scholar 

  6. Sil’chenko, O. K., Moiseev, A. V., Afanasiev, V. L., Chavushyan, V. H. & Valdes, J. R. The Leo I cloud: Secular nuclear evolution of NGC 3379, NGC 3384, and NGC3368? Astrophys. J. 591, 185–203 (2003)

    ADS  Article  Google Scholar 

  7. Rood, H. J. & Williams, B. A. The intergalactic HI cloud in Leo – a simple modelling of the Spitzer-Baade collision event. Astrophys. J. 288, 535–550 (1985)

    ADS  CAS  Article  Google Scholar 

  8. Spitzer, L. & Baade, W. Stellar populations and collisions of galaxies. Astrophys. J. 113, 413–418 (1951)

    ADS  Article  Google Scholar 

  9. Bekki, K., Koribalski, B. S., Ryder, S. D. & Couch, W. J. Massive HI clouds with no optical counterparts as high-density regions of intragroup HI rings and arcs. Mon. Not. R. Astron. Soc. 357, L21–L25 (2005)

    ADS  Article  Google Scholar 

  10. Schneider, S. E. Neutral hydrogen in the M96 group – the galaxies and the intergalactic ring. Astrophys. J. 343, 94–106 (1989)

    ADS  CAS  Article  Google Scholar 

  11. Reynolds, R. J., Magee, K., Roesler, F. L., Scherb, F. & Harlander, J. H-alpha scans of the intergalactic HI cloud in Leo. Astrophys. J. 309, L9–L12 (1986)

    ADS  CAS  Article  Google Scholar 

  12. Donahue, M., Aldering, G. & Stocke, J. T. Low surface brightness H-alpha observations of local intergalactic hydrogen clouds. Astrophys. J. 450, L45–L49 (1995)

    ADS  CAS  Article  Google Scholar 

  13. Martin, D. C. et al. The Galaxy Evolution Explorer: a space ultraviolet survey mission. Astrophys. J. 619, L1–L6 (2005)

    ADS  CAS  Article  Google Scholar 

  14. Bruzual, G. & Charlot, S. Stellar population synthesis at the resolution of 2003. Mon. Not. R. Astron. Soc. 344, 1000–1028 (2003)

    ADS  Article  Google Scholar 

  15. Gil de Paz, A. et al. Chemical and photometric evolution of extended ultraviolet disks: optical spectroscopy of M83 (NGC 5236) and NGC 4625. Astrophys. J. 661, 115–134 (2007)

    ADS  CAS  Article  Google Scholar 

  16. Meurer, G. et al. Evidence for a non-uniform initial mass function in the local Universe. Astrophys. J. (in the press)

  17. Thilker, D. A. et al. A search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Astrophys. J. Suppl. Ser. 173, 538–571 (2007)

    ADS  CAS  Article  Google Scholar 

  18. Boissier, S. et al. Radial variation of attenuation and star formation in the largest late-type disks observed with GALEX. Astrophys. J. Suppl. Ser. 173, 524–537 (2007)

    ADS  CAS  Article  Google Scholar 

  19. Mihos, J. C., Harding, P., Feldmeier, J. & Morrison, H. Diffuse light in the Virgo Cluster. Astrophys. J. 631, L41–L44 (2005)

    ADS  Article  Google Scholar 

  20. Aloisi, A. et al. I Zw 18 revisited with HST ACS and Cepheids: new distance and age. Astrophys. J. 667, L151–L154 (2007)

    ADS  CAS  Article  Google Scholar 

  21. Ryan-Weber, E. V. et al. The local group dwarf Leo T: HI on the brink of star formation. Mon. Not. R. Astron. Soc. 384, 535–540 (2008)

    ADS  CAS  Article  Google Scholar 

  22. de Jong, J. T. A. et al. The structural properties and star formation history of Leo T from deep LBT photometry. Astrophys. J. 680, 1112–1119 (2008)

    ADS  Article  Google Scholar 

  23. Bournaud, F. & Duc, P.-A. From tidal dwarf galaxies to satellite galaxies. Astron. Astrophys. 456, 481–492 (2006)

    ADS  Article  Google Scholar 

  24. Schneider, S. Neutral hydrogen in the M96 group – evidence for a giant intergalactic ring. Astrophys. J. 288, L33–L35 (1985)

    ADS  CAS  Article  Google Scholar 

  25. Millis, R. L. et al. The Deep Ecliptic Survey: a search for Kuiper Belt Objects and Centaurs. I. Description of methods and initial results. Astron. J. 123, 2083–2109 (2002)

    ADS  Article  Google Scholar 

  26. Adelman-McCarthy, J. K. et al. The sixth data release of the Sloan Digital Sky Survey. Astrophys. J. Suppl. Ser. 175, 297–313 (2008)

    ADS  CAS  Article  Google Scholar 

  27. Schlegel, D. J., Finkbeiner, D. P. & Davis, M. Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525–553 (1998)

    ADS  Article  Google Scholar 

  28. Cardelli, J. A., Clayton, G. C. & Mathis, J. S. The relationship between infrared, optical, and ultraviolet extinction. Astrophys. J. 345, 245–256 (1989)

    ADS  CAS  Article  Google Scholar 

  29. O’Donnell, J. E. R ν-dependent optical and near-ultraviolet extinction. Astrophys. J. 422, 158–163 (1994)

    ADS  Article  Google Scholar 

Download references


GALEX is a NASA Small Explorer, launched in April 2003. We gratefully acknowledge NASA’s support of the construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d’Etudes Spatiales, France, and the Korean Ministry of Science and Technology. This research draws upon data provided by B. Millis as distributed by the National Optical Astronomy Observatory (NOAO) Science Archive. NOAO is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. This research made use of NASA’s Astrophysics Data System.

Author information

Authors and Affiliations


Corresponding author

Correspondence to David A. Thilker.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Thilker, D., Donovan, J., Schiminovich, D. et al. Massive star formation within the Leo ‘primordial’ ring. Nature 457, 990–993 (2009).

Download citation

  • Received:

  • Accepted:

  • 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