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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.

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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.


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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.

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Correspondence to David A. Thilker.

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

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