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 ring (1,2), a massive ([M.sub.H I] ≅ 1.8 x [10.sup.9] [M.sub.[dot encircle]], [M.sub.[dot encircle]] 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 ([~10.sup.8]-yr) duration. Measured ultraviolet-visible photometry favours models with low metallicity (Z ≅ [Z.sub.[dot encircle]]/ 50-[Z.sub.[dot encircle]]/5, [Z.sub.[dot encircle]] 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 component (5). 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.
The Leo ring was serendipitously discovered around 25 years ago (1) but remains puzzling today. The most viable interpretation of the ring (Fig. 1) characterizes it as a primordial structure [...]