But what about...: cosmic rays, magnetic fields, conduction, and viscosity in galaxy formation
But what about...: cosmic rays, magnetic fields, conduction, and viscosity in galaxy formation
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DOI：
10.1093/mnras/stz3321
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发表时间：
2019-05
影响因子：
4.8
通讯作者：
P. Hopkins;T. K. Chan;S. Garrison-Kimmel;Suoqing Ji;Kung-Yi Su;C. Hummels;D. Keres̆;E. Quataert;C. Faucher-Giguère
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作者：
P. Hopkins;T. K. Chan;S. Garrison-Kimmel;Suoqing Ji;Kung-Yi Su;C. Hummels;D. Keres̆;E. Quataert;C. Faucher-Giguère
我们提出并研究了大量的高分辨率宇宙学放大模拟，使用FIRE-2处理来自大质量恒星的机械和辐射反馈，以及明确处理磁场，各向异性传导和粘度（考虑高β时等离子体不稳定性的饱和和限制），以及超新星冲击中注入的宇宙射线（cr）（包括各向异性扩散，流，绝热，强子和库仑损失）。我们调查了从超矮星系（$M_{\ast }\sim 10^{4}\, \mathrm{M}_{\odot }$, $M_{\rm halo}\sim 10^{9}\, \mathrm{M}_{\odot }$）到银河系/本星系群（MW/LG）质量的星系，系统地改变了不确定的CR参数（例如扩散系数κ和流速度），并研究了星系特性的广泛集合[质量，恒星形成（SF）历史，质量剖面，相结构，形态等]。我们证实了先前的结论，即在分辨率（$\gtrsim 1\,$ pc）尺度上的磁场、传导和黏度对星系的性质只有很小的影响。在任何合理的物理参数下，cr对矮星（$M_{\ast } \ll 10^{10}\, \mathrm{M}_{\odot }$, $M_{\rm halo} \lesssim 10^{11}\, \mathrm{M}_{\odot }$）或高红移（z > 1-2）中所研究的所有星系特性的影响相对较弱。然而，在更高质量（$M_{\rm halo} \gtrsim 10^{11}\, \mathrm{M}_{\odot }$）和z≤1-2时，如果注入效率合理，有效扩散系数相对较高$\kappa \gtrsim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$， cr可以通过因子～ 2-4抑制SF和恒星质量。在较低的κ下，CRs需要很长时间才能逃离密集的恒星形成气体，并因碰撞强子损失而失去能量，对星系的影响可以忽略不计，并且违反了散裂和γ射线发射的经验约束。在高得多的κ CRs逃逸效率太高，即使在CGM中也没有明显的影响。但在$\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$附近，cr从星系中逃逸出来，形成了一个以cr压力为主的光晕，保持了近似的维里平衡，并支持相对致密、凉爽（T≪106 K）的气体，否则这些气体就会像雨一样落在星系中。CR“加热”（来自碰撞和流损失）从未占主导地位。
We present and study a large suite of high-resolution cosmological zoom-in simulations, using the FIRE-2 treatment of mechanical and radiative feedback from massive stars, together with explicit treatment of magnetic fields, anisotropic conduction and viscosity (accounting for saturation and limitation by plasma instabilities at high β), and cosmic rays (CRs) injected in supernovae shocks (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultrafaint dwarf ($M_{\ast }\sim 10^{4}\, \mathrm{M}_{\odot }$, $M_{\rm halo}\sim 10^{9}\, \mathrm{M}_{\odot }$) through Milky Way/Local Group (MW/LG) masses, systematically vary uncertain CR parameters (e.g. the diffusion coefficient κ and streaming velocity), and study a broad ensemble of galaxy properties [masses, star formation (SF) histories, mass profiles, phase structure, morphologies, etc.]. We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($\gtrsim 1\,$ pc) scales have only small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($M_{\ast } \ll 10^{10}\, \mathrm{M}_{\odot }$, $M_{\rm halo} \lesssim 10^{11}\, \mathrm{M}_{\odot }$), or at high redshifts (z ≳ 1–2), for any physically reasonable parameters. However, at higher masses ($M_{\rm halo} \gtrsim 10^{11}\, \mathrm{M}_{\odot }$) and z ≲ 1–2, CRs can suppress SF and stellar masses by factors ∼2–4, given reasonable injection efficiencies and relatively high effective diffusion coefficients $\kappa \gtrsim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$. At lower κ, CRs take too long to escape dense star-forming gas and lose their energy to collisional hadronic losses, producing negligible effects on galaxies and violating empirical constraints from spallation and γ-ray emission. At much higher κ CRs escape too efficiently to have appreciable effects even in the CGM. But around $\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$, CRs escape the galaxy and build up a CR-pressure-dominated halo which maintains approximate virial equilibrium and supports relatively dense, cool (T ≪ 106 K) gas that would otherwise rain on to the galaxy. CR ‘heating’ (from collisional and streaming losses) is never dominant.