暗物质晕的成对速度：用子弹团检验λ冷暗物质模型
Pairwise Velocities of Dark Matter Halos: a Test for the Lambda Cold Dark Matter Model using the Bullet Cluster
摘要：
1 E0657 -56的存在对λ冷暗物质模型提出了挑战。在这里，我们调查的速度分布的暗物质晕对在大N体模拟不同的盒子大小（250 Mpc/h-2 Gpc/h）和分辨率。我们检查统计，如晕质量，成对晕速度（v_{12}），对分离距离。然后，我们将我们的结果与在非宇宙学流体动力学模拟中再现1 E0657 -56的观测性质所需的初始条件（IC）进行比较。我们发现，高速度尾部的v_{12}分布延伸到更大的速度，因为我们增加了盒子的大小。我们还发现，高v_{12}对的数量增加，我们增加了粒子计数和分辨率与一个固定的盒子大小，但是，这种增加主要是由于较低的质量晕，不匹配的观察到的质量1 E0657 -56。我们发现在z=0.0和z~0.5之间，v_{12}分布函数的红移演化不是很强。我们确定了一些对，其v_{12}类似于所需的IC，然而，即使是最好的候选人有错误的晕质量比，或太大的分离。我们的模拟结果表明，在z=0.0、0.296和0.489的情况下，要在2Gpc/h的体积下产生这样的IC是非常困难的^[3]。基于我们的累积v_{12}函数的外推，我们发现，为了产生至少一对类似于所需的v_{12}和观测到的质量1 E0657 -56的晕，我们需要使用（4.48 Gpc/h）^3和2240^3 DM粒子的共动盒大小进行模拟。我们发现，在z=0.489时，发现v_{12} >= 3000 km/s、质量>=10^{14}Msun的晕对的概率为2.76 × 10 ^{-8}。我们的结论是，要么1 E0657 -56是不兼容的和谐LCDM宇宙，或由非宇宙学模拟建议的IC必须修改，以给出一个较低的值v_{12}。
英文摘要：
The existence of 1E0657-56 poses a challenge to the concordance Lambda cold dark matter model. Here we investigate the velocity distribution of dark matter halo pairs in large N-body simulations with differing box sizes (250Mpc/h-2 Gpc/h) and resolutions. We examine statistics such as the halo masses, pairwise halo velocities (v_{12}), and pair separation distances. We then compare our results to the initial conditions (ICs) required to reproduce the observational properties of 1E0657-56 in non-cosmological hydrodynamical simulations. We find that the high velocity tail of the v_{12} distribution extends to greater velocities as we increase the box size. We also find that the number of high-v_{12} pairs increases as we increase the particle count and resolution with a fixed box size, however, this increase is mostly due to lower mass halos which do not match the observed masses of 1E0657-56. We find that the redshift evolution is not very strong for the v_{12} distribution function between z=0.0 and z~0.5. We identify some pairs whose v_{12} resemble the required ICs, however, even the best candidates have either wrong halo mass ratios, or too large separations. Our simulations suggest that it is very difficult to produce such ICs at z=0.0, 0.296,& 0.489 in comoving volumes as large as (2Gpc/h)^3. Based on the extrapolation of our cumulative v_{12} function, we find that one needs a simulation with a comoving box size of (4.48Gpc/h)^3 and 2240^3 DM particles in order to produce at least one pair of halos that resembles the required v_{12} and observed masses of 1E0657-56. We find that the probability of finding a halo pair with v_{12}>=3000km/s and masses >=10^{14}Msun to be 2.76x10^{-8} at z=0.489. We conclude that either 1E0657-56 is incompatible with the concordance LCDM universe, or the ICs suggested by the non-cosmological simulations must be revised to give a lower value of v_{12}.
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