Inefficient star formation in extremely metal poor galaxies

摘要

第一代恒星是从含有很少比氦重的元素（它们被天文学家称为"金厲"）或不含这种元素的气体形成的。这些条件下的成星过程可能与今天在星系中所看到的成星过程很不同。现在，Yong Shi等人利用Herschel Space Observatory的远红外功能，对两个附近年轻星系的尘埃区进行了分析，那里的极端条件与早期宇宙的原始星系中的主导性条件相似。他们对由原子气主导的区域中的尘埃/气体比做了估计，并用它来估计成星区域中的气体数量。他们发现，那里的成星效率只有今天正常的、富含金属的星系中所看到的成星效率的十分之一。这项工作让我们对早期宇宙中恒星是怎样形成的有所认识，也为验证关于早期星系演变的理论模型提供了一个方便的模型。%The first galaxies contain stars born out of gas with few or no 'metals' (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities, and while dust has been used to trace gas in low-metallicity galaxies, low spatial resolution in the far-infrared has typically led to large uncertainties. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.