高红移X射线暴：探测尘埃产生的历史
X-shooting GRBs at high redshift: probing dust production history
摘要：
演化的渐近巨星分支（AGB）恒星和Ia型超新星（SNe）是形成星系星际介质中尘埃的重要元素，特别是碳和铁。然而，它们至少需要一个陀螺才能开始产生这些元素，因此，在高红移时可能会出现尘埃数量或性质的变化。在这项工作中，我们使用消光的伽马射线暴（GRB）余辉作为一种工具，寻找在z>3的尘埃属性的变化。我们使用光谱选择的样品观察到的伽玛暴余辉与VLT/X-shooter仪器，以确定消光曲线的高红移。我们提出了10个新的z>3 X-射手伽玛暴，其中6个是尘埃。结合这些与以前已知的三个z>3的伽玛暴的个别消光曲线，我们发现一个平均消光曲线与SMC-Bar一致。与光谱学上选择的所有红移的伽玛暴的比较表明，在z>3.5时，视觉消光（A_V）下降，没有中度或高度的消光爆发。我们使用模板光谱检查观测偏差，发现高达Z~8的伽玛暴是可检测的X-射手高达A_V~0.3马格。虽然其他的偏差被注意到，一个均匀的低尘埃含量以上Z>3.5表明一个真正的下降，这表明在尘埃性质和/或可用的尘埃积木的过渡。在z<3.5处粉尘含量的显著增加可能是由于第一富碳AGB和Ia型SNe分别产生碳和可能的铁。或者，z>3.5的尘埃降可能是GRB宿主星系的低恒星质量的结果。
英文摘要：
Evolved asymptotic giant branch (AGB) stars and Type Ia supernovae (SNe) are important contributors to the elements that form dust in the interstellar medium of galaxies, in particular, carbon and iron. However, they require at least a Gyr to start producing these elements, therefore, a change in dust quantity or properties may appear at high redshifts. In this work, we use extinction of gamma-ray burst (GRB) afterglows as a tool to look for variations in dust properties at z>3. We use a spectroscopically selected sample of GRB afterglows observed with the VLT/X-shooter instrument to determine extinction curves out to high redshifts. We present ten new z>3 X-shooter GRBs of which six are dusty. Combining these with individual extinction curves of three previously known z>3 GRBs, we find an average extinction curve consistent with the SMC-Bar. A comparison with spectroscopically selected GRBs at all redshifts indicates a drop in visual extinction (A_V) at z>3.5 with no moderate or high extinction bursts. We check for observational bias using template spectra and find that GRBs up to z~8 are detectable with X-shooter up to A_V~0.3 mag. Although other biases are noted, a uniformly low dust content above z>3.5 indicates a real drop, suggesting a transition in dust properties and/or available dust building blocks. The remarkable increase in dust content at z<3.5 could arise due to carbon and possibly iron production by the first carbon-rich AGB and Type Ia SNe, respectively. Alternatively, z>3.5 dust drop could be the result of low stellar masses of GRB host galaxies.
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