氢原子库中形成的大质量恒星：伽马射线暴宿主星系的HI观测
Massive stars formed in atomic hydrogen reservoirs: HI observations of gamma-ray burst host galaxies
摘要：
长伽马射线暴（GRB）是宇宙中能量最大的事件之一，是大质量和短寿命恒星的爆炸，因此它们可以精确定位最近恒星形成的位置。然而，最近发现几个GRB宿主星系缺乏分子气体（H2），被认为是恒星形成的燃料。此外，伽玛射线暴余辉的光谱学意味着分子相仅构成沿伽玛射线暴视线的气体的一小部分。在这里，我们报告了有史以来第一次使用澳大利亚望远镜紧凑阵列对GRB宿主星系进行的21厘米线观测，这意味着高水平的原子氢（HI），这表明原子气体和恒星形成之间的联系比以前认为的更强，恒星形成可能直接由原子气体提供燃料（或具有非常有效的HI至H2转化和分子气体的快速耗尽），如理论上已经证明是可能的。这可能发生在恒星形成开始时的低金属含量气体中，因为气体的冷却（恒星形成所必需的）比HI到H2的转化更快。事实上，大的原子气库，连同低分子气体质量，恒星和尘埃质量是一致的伽玛射线暴主机优先星系，最近开始了恒星形成插曲吸积贫金属气体从星系际介质。这为在低金属丰度环境中形成伽玛暴提供了一条天然途径。气体流入的情况也与伴HI天体的存在相一致，该天体与GRB 060505主机之间没有光学对应物~19 kpc，并且GRB 980425和060505主机的HI质心与这些星系的光学中心不重合，但靠近GRB位置。
英文摘要：
Long gamma-ray bursts (GRBs), among the most energetic events in the Universe, are explosions of massive and short-lived stars, so they pinpoint locations of recent star formation. However, several GRB host galaxies have recently been found to be deficient in molecular gas (H2), believed to be the fuel of star formation. Moreover, optical spectroscopy of GRB afterglows implies that the molecular phase constitutes only a small fraction of the gas along the GRB line-of-sight. Here we report the first ever 21 cm line observations of GRB host galaxies, using the Australia Telescope Compact Array, implying high levels of atomic hydrogen (HI), which suggests that the connection between atomic gas and star formation is stronger than previously thought, with star formation being potentially directly fuelled by atomic gas (or with very efficient HI-to-H2 conversion and rapid exhaustion of molecular gas), as has been theoretically shown to be possible. This can happen in low metallicity gas near the onset of star formation, because cooling of gas (necessary for star formation) is faster than the HI-to-H2 conversion. Indeed, large atomic gas reservoirs, together with low molecular gas masses, stellar and dust masses are consistent with GRB hosts being preferentially galaxies which have very recently started a star formation episode after accreting metal-poor gas from the intergalactic medium. This provides a natural route for forming GRBs in low-metallicity environments. The gas inflow scenario is also consistent with the existence of the companion HI object with no optical counterpart ~19 kpc from the GRB 060505 host, and with the fact that the HI centroids of the GRB 980425 and 060505 hosts do not coincide with optical centres of these galaxies, but are located close to the GRB positions.
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