锻造在FIRE中：低质量矮星系中的尖点、核和重子
Forged in FIRE: cusps, cores, and baryons in low-mass dwarf galaxies
摘要：
我们提出了超高分辨率的宇宙学流体动力学模拟的$M_*\simeq10^{4-6.3}M_{\odot}$矮星系内形成的$M_{v}=10^{9.5-10}M_{\odot}$暗物质晕。我们的模拟依赖于恒星形成反馈的FIRE实现，并以足够高的力和质量分辨率运行，以直接解决本星系群中经典和超暗矮星在~200 pc尺度上的恒星和暗物质结构。由此产生的星系位于$M_*$与$M_{v}$的关系，需要匹配本星系群恒星质量函数。它们有着爆发式的恒星形成历史，并且形成时的半光半径和金属丰度与在相同恒星质量下观察到的当地矮星大致相符。我们证明，这是可能的，以创建一个大的（~1千秒差距）暗物质核心的宇宙学模拟的$M_*\simeq10^{6.5}M_{\odot}$矮星系，居住在$M_{v}=10^{10}M_{\odot}$晕-正是解决太大而不能失败的问题的兴趣的规模。然而，这些大的核心并不是普遍存在的，并且似乎与矮星的恒星形成历史密切相关：暗物质核心在形成恒星较晚（$z\lesssim2$）的系统中最大，在早期的尖点合并时代结束之后。我们的$M_*\simeq10^4M_{\odot}$矮星保留了一个尖状的暗物质晕密度轮廓，几乎完全符合同一系统中只有暗物质运行的情况。尽管在磁场环境中形成，这个质量非常低的矮星具有与银河系超微弱卫星星系密切匹配的可观测特性，包括均匀的老恒星星族（>10 Gyr）。虽然古老，但大多数恒星在再电离后都是超微弱的形式;紫外线场主要用于抑制新鲜气体的吸积，而不是蒸发掉原矮星中已经存在的气体。
英文摘要：
We present ultra-high resolution cosmological hydrodynamic simulations of $M_*\simeq10^{4-6.3}M_{\odot}$ dwarf galaxies that form within $M_{v}=10^{9.5-10}M_{\odot}$ dark matter halos. Our simulations rely on the FIRE implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve stellar and dark matter structure on the ~200 pc scales of interest for classical and ultra-faint dwarfs in the Local Group. The resultant galaxies sit on the $M_*$ vs. $M_{v}$ relation required to match the Local Group stellar mass function. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. We demonstrate that it is possible to create a large (~1 kpc) dark matter core in a cosmological simulation of an $M_*\simeq10^{6.5}M_{\odot}$ dwarf galaxy that resides within an $M_{v}=10^{10}M_{\odot}$ halo -- precisely the scale of interest for resolving the Too Big to Fail problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late ($z\lesssim2$), after the early epoch of cusp building mergers has ended. Our $M_*\simeq10^4M_{\odot}$ dwarf retains a cuspy dark matter halo density profile that matches almost identically that of a dark-matter only run of the same system. Despite forming in a field environment, this very low mass dwarf has observable properties that match closely to those of ultra-faint satellite galaxies of the Milky Way, including a uniformly old stellar population (>10 Gyr). Though ancient, most of the stars in our ultra-faint form after reionization; the UV field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the proto-dwarf.
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