Thilker David A, Donovan Jennifer, Schiminovich David, Bianchi Luciana, Boissier Samuel, de Paz Armando Gil, Madore Barry F, Martin D Christopher, Seibert Mark
Center for Astrophysical Sciences, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Nature. 2009 Feb 19;457(7232):990-3. doi: 10.1038/nature07780.
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, a massive (M(H(i)) approximately 1.8 x 10(9)M[symbol: see text], M[symbol: see text] 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, 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 (approximately 10(8)-yr) duration. Measured ultraviolet-visible photometry favours models with low metallicity (Z approximately Z[symbol: see text]/50-Z[symbol: see text]/5, Z[symbol: see text] 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. 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.
在本星系群中，人们仅发现了少量可能是原初的中性氢原子云（H(i)），这表明此类结构要么已经消散，要么已被电离，要么在数十亿年的时间尺度上形成了恒星族群。狮子座环是一个巨大的（氢原子质量约为1.8×10⁹太阳质量，太阳质量用M☉表示）、宽度为200千秒差距的结构，它以40亿年的周期环绕星系M105和NGC 3384运行，是一个原初云的候选者。尽管经过多次尝试，此前仅通过氢原子发射观测到过它，这表明它没有恒星族群。在此，我们报告在狮子座环的气态子结构中检测到了紫外光，我们将其归因于近期大量恒星的形成。检测到的复合体的紫外颜色为蓝色，这意味着恒星形成爆发或持续中等时长（约10⁸年）的恒星形成已经开始。实测的紫外-可见光测光数据支持低金属丰度（Z约为Z☉/50 - Z☉/5，Z☉表示太阳金属丰度）的模型，即比氦重的元素比例较低，不过还需要光谱确认。我们推测这些复合体是在形成过程中被观测到的矮星系，但它们的独特之处在于缺乏暗物质成分。在这方面，它们类似于潮汐矮星系，尽管没有潮汐剥离之前的富集过程。如果像狮子座环这样的结构在早期宇宙中很常见，它们可能产生了大量尚未被发现的、微弱的、贫金属的、缺乏晕的矮星系。
