但是...星系形成中的宇宙射线、磁场、传导和粘性
But What About... Cosmic Rays, Magnetic Fields, Conduction, & Viscosity in Galaxy Formation
摘要：
我们提出了一套高分辨率的宇宙学模拟，使用FIRE-2反馈物理与明确的处理一起的磁场，各向异性传导和粘度，和宇宙射线（CR）注入超新星（包括各向异性扩散，流，绝热，强子和库仑损失）。我们调查系统从超微弱的矮（$M_{\ast}\sim 10^{4}\,M_{\odot}$，$M_{\rm halo}\sim 10^{9}\,M_{\odot}$）通过银河系的质量，系统地改变CR参数（如扩散系数$κ$和流动速度），并研究星系性质（质量，恒星形成历史，质量分布，相结构，形态）的合奏。我们证实了以前的结论，磁场，传导，并解决（$\gtrsim 1\,$ PC）尺度上的粘度对散装星系的性质有很小的影响。对于任何物理上合理的参数，CR对矮星（$M_{\ast} \ll 10^{10}\,M_{\odot}$，$M_{\rm halo} \lesssim 10^{11}\,M_{\odot}$）或高红移（$z\gtrsim 1-2$）中研究的所有星系性质的影响相对较弱。然而，在较高的质量（$M_{\rm halo} \gtrsim 10^{11}\,M_{\odot}$）和$z\lesssim 1-2$，CR可以抑制恒星形成的因素$\sim 2-4$，给出相对较高的有效扩散系数$κ\gtrsim 3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$。在较低的$κ$下，CR需要很长时间才能逃离致密的恒星形成气体，并将能量损失到强子碰撞中，对星系产生的影响可以忽略不计，并违反了$γ$射线发射的经验约束。但在$κ\sim 3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$周围，CR逃离了星系，并建立了一个CR压力主导的光环，支持密集，凉爽（$T\ll 10^{6}$ K）的气体，否则会下雨到星系。CR加热（来自碰撞和流动损失）从不占主导地位。
英文摘要：
We present a suite of high-resolution cosmological simulations, using the FIRE-2 feedback physics together with explicit treatment of magnetic fields, anisotropic conduction and viscosity, and cosmic rays (CRs) injected by supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultra-faint dwarf ($M_{\ast}\sim 10^{4}\,M_{\odot}$, $M_{\rm halo}\sim 10^{9}\,M_{\odot}$) through Milky Way masses, systematically vary CR parameters (e.g. the diffusion coefficient $κ$ and streaming velocity), and study an ensemble of galaxy properties (masses, star formation histories, mass profiles, phase structure, morphologies). We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($\gtrsim 1\,$pc) scales have small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($M_{\ast} \ll 10^{10}\,M_{\odot}$, $M_{\rm halo} \lesssim 10^{11}\,M_{\odot}$), or at high redshifts ($z\gtrsim 1-2$), for any physically-reasonable parameters. However at higher masses ($M_{\rm halo} \gtrsim 10^{11}\,M_{\odot}$) and $z\lesssim 1-2$, CRs can suppress star formation by factors $\sim 2-4$, given relatively high effective diffusion coefficients $κ\gtrsim 3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$. At lower $κ$, CRs take too long to escape dense star-forming gas and lose energy to hadronic collisions, producing negligible effects on galaxies and violating empirical constraints from $γ$-ray emission. But around $κ\sim 3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$, CRs escape the galaxy and build up a CR-pressure-dominated halo which supports dense, cool ($T\ll 10^{6}$ K) gas that would otherwise rain onto the galaxy. CR heating (from collisional and streaming losses) is never dominant.
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