超级星团NGC1705-1：长持续时间伽马射线暴爆发地点的一个局部观测
Super Star Cluster NGC1705-1: A Local Analogue to the Birthsite of Long-duration Gamma-ray Bursts
摘要：
最近的观测表明，长时间伽马射线暴（GRB）的宿主星系的全球性质特别适合于创造这些GRB的大质量恒星祖先。基于这样一个假设，即位于贫金属、低金属量星系中的大质量年轻星团是GRB祖先的天然诞生地，我们对超级星团NGC 1705 -1和遥远GRB宿主的ISM特性进行了比较研究。使用相同的一套金属跃迁在紫外线和应用已知的ISM结构在NGC 1705，我们发现，NGC 1705 -1类似于遥远的伽玛射线暴宿主星系在其高中性气体柱，低分子气体分数，低金属丰度，α-元素增强，和低尘埃耗尽。分子气体的缺乏是由于在星暴环境中增强的紫外辐射场，与GRB祖先的预期一致。此外，已知NGC 1705 -1中存在r<500 pc的致密中性气体云，这为C^+、Si^+和Fe^+离子提供了一个天然的储存库，这些离子随后可能被余辉紫外辐射场激发，产生在伽玛射线暴宿主ISM中常见的激发线。我们进一步认为，从吸收线观测的低和高电离跃迁的速度剖面的明显偏移已经提供了重要的线索，在伽玛射线暴宿主星系相关的星爆事件。我们的研究表明，统计比较周围的星团和高红移伽玛暴祖星的ISM是很重要的识别的关键物理参数，促进伽玛暴的形成。
英文摘要：
Recent observations suggest that global properties of the host galaxies for long-duration gamma-ray bursts (GRBs) are particularly well-suited for creating the massive star progenitors of these GRBs. Motivated by the hypothesis that massive young star clusters located in metal-poor, low-metallicity galaxies are a natural birthsite for GRB progenitors, we present a comparison study of the ISM properties along the sightline toward the super star cluster, NGC1705-1, and those in distant GRB hosts. Using the same set of metal transitions in the UV and applying known ISM structures in NGC1705, we find that NGC1705-1 resembles distant GRB host galaxies in its high neutral gas column, low molecular gas fraction, low metallicity, alpha-element enhancement,and low dust depletion. The lack of molecular gas is due to the enhanced UV radiation field in the starburst environment, consistent with the expectations for GRB progenitors. In addition, the known presence of dense neutral gas clouds at r<500 pc from NGC1705-1 provides a natural reservoir of C^+, Si^+, and Fe^+ ions that may be subsequently excited by the afterglow UV radiation field to produce excited lines commonly seen in GRB host ISM. We further argue that the apparent offset in the velocity profiles of low- and high-ionization transitions from absorption-line observations alone already offers important clues for related starburst episodes in GRB host galaxies. Our study shows that a statistical comparison between the ISM around star clusters and high-redshift GRB progenitors is important for identifying the key physical parameters that facilitate the formation of GRBs.
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