How low does it go? Too few Galactic satellites with standard reionization quenching
How low does it go? Too few Galactic satellites with standard reionization quenching
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DOI：
10.1093/mnras/stz1992
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发表时间：
2018-08
影响因子：
4.8
通讯作者：
A. Graus;J. Bullock;Tyler Kelley;M. Boylan-Kolchin;S. Garrison-Kimmel;Yuewen Qi
中科院分区：
文献类型：
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作者：
A. Graus;J. Bullock;Tyler Kelley;M. Boylan-Kolchin;S. Garrison-Kimmel;Yuewen Qi
星系形成理论的一个标准预测是，电离背景抑制了星系形成的晕，其峰值圆速度小于$V_{\rm peak}\simeq 20 \，\rm km \，s^{-1}$，使大多数低于这个尺度的晕完全黑暗。我们使用一套宇宙学变焦模拟银河系的晕，包括中央银河系盘星系的潜力，调查subhaloes和ultrafeak星系之间的关系。我们发现，在银河系50千秒差距内，V_{\rm peak}\gtrsim 20\，\rm km \，s^{-1}$的次晕太少了，无法解释今天已知的超暗星系的数量。为了与观测到的数量相匹配，我们必须用超微弱的矮星填充低至V_{\rm peak}\simeq 6\，\rm km \，s^{-1}$的亚晕。所需的晕具有低至1500 K的峰值维里温度，远低于104 K的原子氢冷却极限。考虑到大麦哲伦云可能在50 kpc范围内贡献几颗卫星，这可能会将这个阈值提高到10 km，s^{-1}$（4000 K），仍然低于原子冷却极限，远低于标称的再电离阈值。
A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than $V_{\rm peak}\simeq 20 \, \rm km \, s^{-1}$, rendering the majority of haloes below this scale completely dark. We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disc galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies. We find that there are far too few subhaloes within 50 kpc of the Milky Way that had $V_{\rm peak}\gtrsim 20\, \rm km \, s^{-1}$ to account for the number of ultrafaint galaxies already known within that volume today. In order to match the observed count, we must populate subhaloes down to $V_{\rm peak}\simeq 6\, \rm km \, s^{-1}$ with ultrafaint dwarfs. The required haloes have peak virial temperatures as low as 1500 K, well below the atomic hydrogen cooling limit of 104 K. Allowing for the possibility that the Large Magellanic Cloud contributes several of the satellites within 50 kpc could potentially raise this threshold to $10\, \rm km \, s^{-1}$ (4000 K), still below the atomic cooling limit and far below the nominal reionization threshold.