1. Center for Astrophysical Sciences, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA
2. Department of Astronomy, Columbia University, Pupin Physics Laboratories, Mail Code 5246, 550 West 120th Street, New York, New York 10027, USA
3. Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France
4. Departamento de Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
5. Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, California 91101, USA
6. California Institute of Technology, MC 405-47, 1200 East California Boulevard, Pasadena, California 91125, USA

Correspondence to: David A. Thilker¹ Correspondence and requests for materials should be addressed to D.A.T. (Email: dthilker@pha.jhu.edu).

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 ring^{1, 2}, a massive (M _H 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 atttempts^{3, 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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