The Lazy Giants: APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds
The Lazy Giants: APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds
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DOI：
10.3847/1538-4357/ab7305
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发表时间：
2019-01
期刊：
影响因子：
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通讯作者：
D. Nidever;S. Hasselquist;C. Hayes;K. Hawkins;Joshua Povick;S. Majewski;V. Smith;B. Anguiano
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文献类型：
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作者：
D. Nidever;S. Hasselquist;C. Hayes;K. Hawkins;Joshua Povick;S. Majewski;V. Smith;B. Anguiano
我们首次在远地点测量了3600颗红巨星的金属丰度和α-元素丰度，这些红巨星横跨大（LMC）和小麦哲伦云（最大的银河系（MW）矮星系）的大径向范围。我们的样本比以前的研究大一个数量级，并且延伸到更大的径向距离。这是利用拉斯坎帕纳斯天文台杜邦望远镜上新安装的南方远地点仪器所获得的第一批结果。我们对麦哲伦云的无偏取样涵盖了很大的金属丰度范围，从[Fe/H] = - 0.2到[Fe/H]≈- 2.5的极贫金属恒星，这是迄今为止探测到的最贫金属的麦哲伦云（MC）恒星。LMC [α/Fe] - [Fe/H]分布在较大的金属丰度范围内非常平坦，但在- 1.0 < [Fe/H] > - 0.5时上升了约0.1个指数。我们将此解释为最近已知的MC恒星形成活动增加的迹象，并能够用化学演化模型重现这种模式，其中包括最近的“星暴”。在贫金属端，我们捕捉到[α/Fe]随着[Fe/H]的降低而增加，并且在两个mc中约束“α-膝”为[Fe/H] > - 2.2，这意味着低的恒星形成效率为~ 0.01 Gyr−1。与天炉座、雕刻家或人马座等质量较小的MW矮星系相比，MC的膝盖更缺乏金属。一种可能的解释是，中子星形成于密度比中子星低的环境中，这一假设与中子星最近才落入中子星引力势的范式相一致。
We report the first APOGEE metallicities and α-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = −0.2 to very metal-poor stars with [Fe/H] ≈ −2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [α/Fe]–[Fe/H] distribution is very flat over a large metallicity range but rises by ∼0.1 dex at −1.0 < [Fe/H] ≲ −0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent “starburst.” At the metal-poor end, we capture the increase of [α/Fe] with decreasing [Fe/H] and constrain the “α-knee” to [Fe/H] ≲ −2.2 in both MCs, implying a low star formation efficiency of ∼0.01 Gyr−1. The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW’s gravitational potential only recently.