arXiv 2210.142322022-10astro-ph.GAastro-ph.HE

用环银河系介质观测约束Cosmic-ray输运

Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium

Iryna S. Butsky, Shreya Nakum, Sam B. Ponnada, Cameron B. Hummels, Suoqing Ji, Philip F. Hopkins

备注：: 9 pages, 2 figures, accepted to MNRAS
DOI：: 10.1093/mnras/stad671

摘要：

最近的理论研究预测，低红移的$\sim L_*$星系周围的环星系介质（CGM）可能有大量的非热压力支持的形式宇宙射线。然而，这些预测是敏感的宇宙射线传输的具体模型，这是理论和观测不足的约束。在这项工作中，我们提出了一个新的观测约束计算的径向平均，有效的宇宙射线传输率，$κ_{\rm min}^{\rm eff}$的下限。在广泛的假设下（只要宇宙射线在银盘中不损失其能量的显著部分，无论宇宙射线压力在CGM中是否重要），我们证明了$κ_{\rm min}^{\rm eff}$和三个可观测的星系属性之间的明确关系：总氢柱密度，平均恒星形成率和气体圆周速度。我们使用一套FIRE-2星系模拟与各种宇宙射线传输物理证明，我们的分析模型的$κ_{\rm min}^{\rm eff}$是一个强大的下限的真实宇宙射线传输率。然后，我们应用我们的新模型来计算星系中的COS-Halos样本的$κ_{\rm min}^{\rm eff}$，并确认这已经揭示了有力的证据，有效的传输率迅速上升远离星际介质的值$κ_{\rm min}^{\rm eff}\gtrsim 10^{30-31}\,{\rm cm}^2\,{\rm s}^{-1}$（对应于各向异性流速度的$v^{\rm stream}_{\rm eff} \gtrsim 1000\,{\rm km}\,{\rm s}^{-1}$）在弥漫CGM，在碰撞参量大于$50-100$ \，kpc。我们将讨论如何在未来的观测可以提供我们的宇宙射线在CGM和星系际介质的理解定性新的约束。

英文摘要：

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, $\sim L_*$ galaxies could have substantial nonthermal pressure support in the form of cosmic rays. However, these predictions are sensitive to the specific model of cosmic-ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a novel observational constraint for calculating the lower limit of the radially-averaged, effective cosmic-ray transport rate, $κ_{\rm min}^{\rm eff}$. Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disk, regardless of whether the cosmic-ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between $κ_{\rm min}^{\rm eff}$ and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of FIRE-2 galaxy simulations with a variety of cosmic-ray transport physics to demonstrate that our analytic model of $κ_{\rm min}^{\rm eff}$ is a robust lower limit of the true cosmic-ray transport rate. We then apply our new model to calculate $κ_{\rm min}^{\rm eff}$ for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate which rises rapidly away from the interstellar medium to values $κ_{\rm min}^{\rm eff}\gtrsim 10^{30-31}\,{\rm cm}^2\,{\rm s}^{-1}$ (corresponding to anisotropic streaming velocities of $v^{\rm stream}_{\rm eff} \gtrsim 1000\,{\rm km}\,{\rm s}^{-1}$) in the diffuse CGM, at impact parameters larger than $50-100$\,kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

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