各向异性热传导下团簇形成的宇宙学MHD模拟
Cosmological MHD simulations of cluster formation with anisotropic thermal conduction
摘要：
在存在磁场和各向异性热传导的情况下，ICM被认为是不稳定的。我们进行第一宇宙学模拟的星系团簇形成，同时包括磁场，辐射冷却和各向异性热传导。在孤立和理想化的团簇模型中，磁热不稳定性（MTI）倾向于径向重定向磁场。使用宇宙学模拟的圣巴巴拉集群，我们检测径向偏置的速度和磁场。这种径向偏置与由于子结构或即使在湍流存在下也预期的残余MTI驱动场重排引起的不均匀径向气流一致。虽然解开这两种情况是具有挑战性的，我们没有检测到过度的偏见，包括各向异性热传导的运行。通过LOFAR和SKA的无线电极化测量以及未来IXO的X射线光谱研究，各向异性效应可能是可检测的。我们表明，辐射冷却提高了放大的磁场约两个数量级超出预期的非辐射的情况下。在z=0时，磁场与仅由宇宙膨胀产生的均匀磁场相比被放大了约10^6倍。有趣的是，运行，包括辐射冷却和各向异性热传导表现出更强的磁场放大比纯粹的辐射运行在偏离中心的位置。在这些运行中，远离星团中心的浅温度梯度使ICM中性浮力。ICM在这些区域中更容易混合，并且冻结的磁场的缠绕更有效，从而导致更强的磁场放大。
英文摘要：
(abridged) The ICM has been suggested to be buoyantly unstable in the presence of magnetic field and anisotropic thermal conduction. We perform first cosmological simulations of galaxy cluster formation that simultaneously include magnetic fields, radiative cooling and anisotropic thermal conduction. In isolated and idealized cluster models, the magnetothermal instability (MTI) tends to reorient the magnetic fields radially. Using cosmological simulations of the Santa Barbara cluster we detect radial bias in the velocity and magnetic fields. Such radial bias is consistent with either the inhomogeneous radial gas flows due to substructures or residual MTI-driven field rearangements that are expected even in the presence of turbulence. Although disentangling the two scenarios is challenging, we do not detect excess bias in the runs that include anisotropic thermal conduction. The anisotropy effect is potentially detectable via radio polarization measurements with LOFAR and SKA and future X-ray spectroscopic studies with the IXO. We demonstrate that radiative cooling boosts the amplification of the magnetic field by about two orders of magnitude beyond what is expected in the non-radiative cases. At z=0 the field is amplified by a factor of about 10^6 compared to the uniform magnetic field evolved due to the universal expansion alone. Interestingly, the runs that include both radiative cooling and anisotropic thermal conduction exhibit stronger magnetic field amplification than purely radiative runs at the off-center locations. In these runs, shallow temperature gradients away from the cluster center make the ICM neutrally buoyant. The ICM is more easily mixed in these regions and the winding up of the frozen-in magnetic field is more efficient resulting in stronger magnetic field amplification.
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