年轻宇宙中正常大质量恒星形成星系中的高分子气体分数
High molecular gas fractions in normal massive star forming galaxies in the young Universe
摘要：
恒星是由寒冷的星际气体分子形成的。由于这种情况在本地宇宙中相对罕见，像银河系这样的星系每年只形成几颗新恒星。遥远宇宙中典型的大质量星系形成恒星的速度要快一个数量级。除非恒星形成的效率明显更高，否则这种差异表明年轻星系的气体丰富得多。到目前为止，对遥远宇宙中分子气体的观测主要局限于非常明亮的稀有天体，包括合并和类星体。在这里，我们报告了一个典型的大质量恒星形成星系的样本中的分子气体的系统调查的结果在<z>~1.2和2.3，当宇宙是40%和24%的当前年龄。我们的测量提供了经验证据，遥远的恒星形成星系确实是气体丰富，恒星形成效率并不强烈依赖于宇宙时代。在z= 2.3和z=1.2时，冷气相对于星系重子总质量的平均比例分别约为44%和34%，比今天的大质量螺旋星系高出3到10倍。z~2和1之间的缓慢减小可能需要一种半连续的新鲜气体补充到年轻星系的机制。
英文摘要：
Stars form from cold molecular interstellar gas. Since this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more gas rich. Molecular gas observations in the distant Universe have so far been largely restricted to very luminous, rare objects, including mergers and quasars. Here we report the results of a systematic survey of molecular gas in samples of typical massive star forming galaxies at <z>~1.2 and 2.3, when the Universe was 40% and 24% of its current age. Our measurements provide empirical evidence that distant star forming galaxies indeed were gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z= 2.3 and z=1.2 is ~44% and 34%, three to ten times higher than in today's massive spiral galaxies. The slow decrease between z~2 and 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.
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