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arXiv 1612.02001astro-ph.GAastro-ph.CO
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一个巨大的,静止的,人口II星系,红移为2.1

A massive, quiescent, population II galaxy at a redshift of 2.1

Mariska Kriek, Charlie Conroy, Pieter G. van Dokkum, Alice E. Shapley, Jieun Choi, Naveen A. Reddy, Brian Siana, Freeke van de Voort, Alison L. Coil, Bahram Mobasher

摘要:

与银河系等螺旋星系不同,大质量椭圆星系中的大多数恒星都是在宇宙历史早期的短时间内形成的。这个形成期的持续时间可以用镁与铁的丰度比([Mg/Fe])来衡量,它反映了核心坍塌和Ia型超新星的相对富集。对于本地星系,[Mg/Fe]探测了目前星系中所有恒星的合并形成历史,包括在后期合并过程中添加的年轻和贫金属恒星。因此,为了直接限制初始恒星形成时期,我们必须研究早期的星系。之前测量到的最远星系的[Mg/Fe]在z~1.4,[Mg/Fe]=0.45(+0.05,-0.19)。通过叠加24个大质量静止星系的光谱,探测到了一个稍早的时代(z~1.6),得到平均[Mg/Fe]为0.31+/-0.12。然而,相对较低的S/N的数据和指数分析技术的使用,这两项研究导致测量误差太大,让我们形成强有力的结论。需要更早时期的更深光谱与基于全光谱拟合的分析技术相结合,才能在主要恒星形成阶段(z>2)后不久精确测量丰度模式。在这里,我们报告的测量[Mg/Fe]的一个巨大的静止星系在z=2.1。该星系的[Mg/Fe]=0.59+/-0.11,是迄今为止发现的Mg增强最多的大质量星系,是今天类似质量星系Mg增强的两倍。该星系的丰度模式与核心坍缩超新星的富集完全一致,恒星形成时间尺度为0.1-0.5 Gyr -类似于银河系中的第二星族恒星。平均过去的SFR为600-3000 Msol/年,这个星系是宇宙中最活跃的恒星形成星系之一。

英文摘要:

Unlike spiral galaxies such as the Milky Way, the majority of the stars in massive elliptical galaxies were formed in a short period early in the history of the Universe. The duration of this formation period can be measured using the ratio of magnesium to iron abundance ([Mg/Fe]), which reflects the relative enrichment by core-collapse and type Ia supernovae. For local galaxies, [Mg/Fe] probes the combined formation history of all stars currently in the galaxy, including younger and metal-poor stars that were added during late-time mergers. Therefore, to directly constrain the initial star-formation period, we must study galaxies at earlier epochs. The most distant galaxy for which [Mg/Fe] had previously been measured is at z~1.4, with [Mg/Fe]=0.45(+0.05,-0.19). A slightly earlier epoch (z~1.6) was probed by stacking the spectra of 24 massive quiescent galaxies, yielding an average [Mg/Fe] of 0.31+/-0.12. However, the relatively low S/N of the data and the use of index analysis techniques for both studies resulted in measurement errors that are too large to allow us to form strong conclusions. Deeper spectra at even earlier epochs in combination with analysis techniques based on full spectral fitting are required to precisely measure the abundance pattern shortly after the major star-forming phase (z>2). Here we report a measurement of [Mg/Fe] for a massive quiescent galaxy at z=2.1. With [Mg/Fe]=0.59+/-0.11, this galaxy is the most Mg-enhanced massive galaxy found so far, having twice the Mg enhancement of similar-mass galaxies today. The abundance pattern of the galaxy is consistent with enrichment exclusively by core-collapse supernovae and with a star-formation timescale of 0.1-0.5 Gyr - characteristics that are similar to population II stars in the Milky Way. With an average past SFR of 600-3000 Msol/yr, this galaxy was among the most vigorous star-forming galaxies in the Universe.

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