Dancing in the Dark: Uncertainty in Ultrafaint Dwarf Galaxy Predictions from Cosmological Simulations
Dancing in the Dark: Uncertainty in Ultrafaint Dwarf Galaxy Predictions from Cosmological Simulations
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DOI：
10.3847/1538-4357/ab0085
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发表时间：
2018-10
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影响因子：
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通讯作者：
F. Munshi;A. Brooks;C. Christensen;Elaad Applebaum;K. Holley-Bockelmann;T. Quinn;J. Wadsley
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文献类型：
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作者：
F. Munshi;A. Brooks;C. Christensen;Elaad Applebaum;K. Holley-Bockelmann;T. Quinn;J. Wadsley
超微弱矮星系（UFD）的存在凸显了将我们对星系的理论理解推向极低质量的必要性。我们通过两次运行矮星系的完全宇宙学模拟来检查 UFD 的形成，但改变了恒星的形成。一次运行使用恒星形成的温度-密度阈值，而另一次运行使用基于 H2 的亚网格恒星形成模型。两次运行之间形成的矮星系总数相差 2 倍，但其中大多数是卫星，导致更大质量、孤立的矮星周围的发光 UFD 伴星数量相差 5 倍。第一次运行有 47% 的机会找到 Mhalo ∼ 1010 M⊙ 主机周围的卫星，而 H2 运行预测只有 16% 的机会。金属丰度是造成这种差异的主要物理参数。随着金属丰度的降低，氢气的形成速度减慢并被降级为密度较高的材料。因此，在再电离去除气体之前，我们的氢气运行无法形成许多（通常是任何）恒星。这些结果强调，使用流体动力学模拟对 UFD 特性进行的预测，特别是关于矮星系周围卫星的频率、低质量恒星质量函数的斜率以及占据最小晕的超微弱星系的特性，对模拟中包含的恒星形成的亚网格物理极其敏感。然而，即将发现的 UFD 将为第一颗恒星形成的物理学提供宝贵的约束力。
The existence of ultrafaint dwarf (UFD) galaxies highlights the need to push our theoretical understanding of galaxies to extremely low mass. We examine the formation of UFDs by twice running a fully cosmological simulation of dwarf galaxies, but varying star formation. One run uses a temperature–density threshold for star formation, while the other uses an H2-based subgrid star formation model. The total number of dwarf galaxies that form is different by a factor of 2 between the two runs, but most of these are satellites, leading to a factor of 5 difference in the number of luminous UFD companions around more massive, isolated dwarfs. The first run yields a 47% chance of finding a satellite around an Mhalo ∼ 1010 M⊙ host, while the H2 run predicts only a 16% chance. Metallicity is the primary physical parameter that creates this difference. As metallicity decreases, the formation of H2 is slowed and relegated to higher-density material. Thus, our H2 run is unable to form many (and often, any) stars before reionization removes gas. These results emphasize that predictions for UFD properties made using hydrodynamic simulations, in particular regarding the frequency of satellites around dwarf galaxies, the slope of the stellar mass function at low masses, and the properties of ultrafaint galaxies occupying the smallest halos, are extremely sensitive to the subgrid physics of star formation contained within the simulation. However, upcoming discoveries of UFDs will provide invaluable constraining power on the physics of the first star formation.