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关于Sunyaev-Zel'dovich和X射线巡天III的星系团物理：气体和恒星质量分数的测量偏差和宇宙学演化

On the Cluster Physics of Sunyaev-Zel'dovich and X-ray Surveys III: Measurement Biases and Cosmological Evolution of Gas and Stellar Mass Fractions

N. Battaglia, J. R. Bond, C. Pfrommer, J. L. Sievers

备注：
Accepted by ApJ, 24 pages, 17 figures, this version additionally includes projection effects, an improved discussion of hydrostatic equilibrium masses, and compares our simulation results to more data
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摘要：

气体质量与星系团的维里质量密切相关，这使得通过大尺度X射线巡天可以精确测定宇宙学参数。然而，根据最近的朱雀X射线测量，气体质量分数，f_gas，似乎大大大于宇宙平均维里半径，R_200，质疑宇宙学参数估计的准确性。在这里，我们使用一个大的宇宙学流体动力学模拟套件来研究f_gas的测量偏差。我们采用不同的变种模拟物理，包括辐射气体物理，恒星形成和热反馈的活动星系核。计算质量分布在48个角锥，其足迹分区的球体，我们发现各向异性的气体和总质量分布，意味着角方差的f_气体在30%的水平。这种各向异性的分布起源于最近的形成时代的集群和从强大的内部重子暗物质的密度偏差。在最极端的锥中，f_gas可以在大质量星系团的R_200处偏高两倍，从而为朱雀的高f_gas测量提供了一个潜在的解释。虽然投影降低了这个因素，但还有其他测量偏差可以（部分）补偿。我们发现，在R_200，f_gas偏高20%时，假设流体静力平衡质量，即，忽略动压，以及由于密度聚集的存在而增加约10-20%。在更大的半径，两个测量偏差急剧增加。虽然真实f_gas的聚类样本方差在R_200处降低到5%的水平，但包括两个测量偏差的样本方差在10- 20%的水平上保持相当恒定。大质量星系团在R_500范围内f_gas的不变红移演化对于利用气体质量推导宇宙学参数是令人鼓舞的。

英文摘要：

Gas masses tightly correlate with the virial masses of galaxy clusters, allowing for a precise determination of cosmological parameters by means of large-scale X-ray surveys. However, according to recent Suzaku X-ray measurements, gas mass fractions, f_gas, appear to be considerably larger than the cosmic mean at the virial radius, R_200, questioning the accuracy of the cosmological parameter estimations. Here, we use a large suite of cosmological hydrodynamical simulations to study measurement biases of f_gas. We employ different variants of simulated physics, including radiative gas physics, star formation, and thermal feedback by active galactic nuclei. Computing the mass profiles in 48 angular cones, whose footprints partition the sphere, we find anisotropic gas and total mass distributions that imply an angular variance of f_gas at the level of 30%. This anisotropic distribution originates from the recent formation epoch of clusters and from the strong internal baryon-to-dark-matter density bias. In the most extreme cones, f_gas can be biased high by a factor of two at R_200 in massive clusters, thereby providing a potential explanation for high f_gas measurements by Suzaku. While projection lowers this factor, there are other measurement biases that may (partially) compensate. We find that at R_200, f_gas is biased high by 20% when assuming hydrostatic equilibrium masses, i.e., neglecting the kinetic pressure, and by another ~10-20% due to the presence of density clumping. At larger radii, both measurement biases increase dramatically. While the cluster sample variance of the true f_gas decreases to a level of 5% at R_200, the sample variance that includes both measurement biases remains fairly constant at the level of 10-20%. The constant redshift evolution of f_gas within R_500 for massive clusters is encouraging for using gas masses to derive cosmological parameters.

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