The Halo Occupation Distribution and the Physics of Galaxy Formation
The Halo Occupation Distribution and the Physics of Galaxy Formation
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DOI：
10.1086/376517
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发表时间：
2002-12
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通讯作者：
A. Berlind;D. Weinberg;A. Benson;C. Baugh;S. Cole;R. Dav'e;C. Frenk;A. Jenkins;N. Katz;C. Lacey
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作者：
A. Berlind;D. Weinberg;A. Benson;C. Baugh;S. Cole;R. Dav'e;C. Frenk;A. Jenkins;N. Katz;C. Lacey
晕占据分布(HOD)通过指定(1)维里质量为M的晕包含特定类别的N个星系的概率P(N|M)和(2)晕内星系和暗物质的相对空间和速度分布来描述星系和暗物质之间的偏差。我们计算并比较了Λ清洁发展机制宇宙学模型(具有宇宙学常数的冷暗物质)的光滑粒子流体动力学模拟和应用于相同宇宙学的半解析星系形成模型所预测的HOD。尽管这两种方法预测的星系质量函数不同，但它们对相同空间密度样本的HOD预测非常一致。在由重子质量阈值定义的样本中，平均占据函数⟨N⟩M在低晕质量时表现出急剧的截止，在近十年的晕质量中⟨N⟩从1攀升到2的缓慢上升平台，以及在高晕质量时更陡峭地上升的高占有率区域。在低占有率区域，阶乘矩⟨N(N-1)⟩和⟨N(N-1)(N-2)⟩远低于泊松统计的⟨N⟩2和⟨N⟩3的预期值，这对两点和三点关联函数的小尺度行为具有重要的影响。HOD强烈依赖于星系年龄，高质量晕主要由老星系占据，低质量晕主要由年轻星系占据。SPH晕内星系的分布支持了通常在半解析计算中做出的假设：最大质量的星系靠近晕中心，运动速度接近晕的平均速度，而其余的卫星星系具有与暗物质相同的径向轮廓和速度弥散。在SPH模拟中，固定晕质量的平均占有率与晕的大尺度环境无关，这既支持半解析方法的合并树方法，也支持HOD提供了星系偏向的完整统计特征的说法。我们讨论了预测的HOD与SPH和半解析方法中包含的星系形成物理之间的联系。这些预测为星系团的理论模型提供了有用的指导，并将通过正在进行的星系红移调查分析进行经验验证。通过将HODS应用于大体积N体模拟，我们表明，这两种方法都预测了与幂定律星系关联函数的轻微偏离，类似于最近的观测分析中检测到的特征。
The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (1) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (2) the relative spatial and velocity distributions of galaxies and dark matter within halos. We calculate and compare the HODs predicted by a smoothed particle hydrodynamics (SPH) simulation of a ΛCDM cosmological model (cold dark matter with a cosmological constant) and by a semianalytic galaxy formation model applied to the same cosmology. Although the two methods predict different galaxy mass functions, their HOD predictions for samples of the same space density agree remarkably well. In a sample defined by a baryonic mass threshold, the mean occupation function ⟨N⟩M exhibits a sharp cutoff at low halo masses, a slowly rising plateau in which ⟨N⟩ climbs from 1 to 2 over nearly a decade in halo mass, and a more steeply rising high-occupancy regime at high halo mass. In the low-occupancy regime, the factorial moments ⟨N(N - 1)⟩ and ⟨N(N - 1)(N - 2)⟩ are well below the values of ⟨N⟩2 and ⟨N⟩3 expected for Poisson statistics, with important consequences for the small-scale behavior of the two- and three-point correlation functions. The HOD depends strongly on galaxy age, with high-mass halos populated mainly by old galaxies and low-mass halos by young galaxies. The distribution of galaxies within SPH halos supports the assumptions usually made in semianalytic calculations: the most massive galaxy lies close to the halo center and moves near the halo's mean velocity, while the remaining, satellite galaxies have the same radial profile and velocity dispersion as the dark matter. The mean occupation at fixed halo mass in the SPH simulation is independent of the halo's larger scale environment, supporting both the merger tree approach of the semianalytic method and the claim that the HOD provides a complete statistical characterization of galaxy bias. We discuss the connections between the predicted HODs and the galaxy formation physics incorporated in the SPH and semianalytic approaches. These predictions offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. By applying the HODs to a large-volume N-body simulation, we show that both methods predict slight departures from a power-law galaxy correlation function, similar to features detected in recent observational analyses.