Halo和星系合并的累积中微子和Gamma-Ray背景
Cumulative Neutrino and Gamma-Ray Backgrounds from Halo and Galaxy Mergers
摘要：
暗物质晕和嵌入其中的气体结构（如原星系、星系、星系团和星系团）的合并，导致了能够将宇宙射线（CR）加速到$\sim10~\rm PeV$的强烈冲击。这些冲击将通过与周围气体环境的非弹性$pp$碰撞产生高能中微子和$γ$ -射线。在这项工作中，我们研究了这些晕合并的贡献，在冰立方测量的扩散中微子流量和非耀变体部分的河外$γ$射线背景测量的$Fermi$。为了计算它们，我们制定的红移依赖的冲击速度，银河半径，晕气体含量和星系/星系际磁场的暗物质晕分布高达红移$z=10$。我们发现，高红移合并贡献了大量的宇宙射线的能量光度密度，由此产生的中微子光谱可以解释很大一部分的观察到的扩散中微子流量超过0.1 PeV的几个PeV。我们还表明，我们的模型可以在一定程度上缓解与河外$γ$射线背景的紧张局势。首先，由于大部分CR能量光度密度来自高红移，伴随的$γ$ -射线通过与宇宙微波背景（CMB）和河外背景光（EBL）的$γγ$湮灭而被更强烈地抑制。其次，轻度辐射冷却激波可能导致更硬的CR谱，谱指数为$1.5\lesssim s\lesssim2.0$。我们的研究表明，晕合并，其中一部分也可能导致合并星系中的星爆，可以是有前途的中微子发射器，而不违反现有的$Fermi$ $γ$射线约束的非耀变体的河外$γ$射线背景的组成部分。
英文摘要：
The merger of dark matter halos and the gaseous structures embedded in them, such as proto-galaxies, galaxies, and groups and clusters of galaxies, results in strong shocks that are capable of accelerating cosmic rays (CRs) to $\sim10~\rm PeV$. These shocks will produce high-energy neutrinos and $γ$-rays through inelastic $pp$ collisions with ambient gaseous environments. In this work, we study the contributions of these halo mergers to the diffuse neutrino flux measured in IceCube and to the non-blazar portion of the extragalactic $γ$-ray background measured by $Fermi$. In order to calculate them, we formulate the redshift dependence of the shock velocity, galactic radius, halo gas content and galactic/intergalactic magnetic fields over the dark matter halo distribution up to a redshift $z=10$. We find that high-redshift mergers contribute a significant amount of the cosmic-ray energy luminosity density, and the resulting neutrino spectra could explain a large part of the observed diffuse neutrino flux above 0.1 PeV up to several PeV. We also show that our model can somewhat alleviate tensions with the extragalactic $γ$-ray background. First, since a larger fraction of the CR energy luminosity density comes from high redshifts, the accompanying $γ$-rays are more strongly suppressed through $γγ$ annihilations with the cosmic microwave background (CMB) and the extragalactic background light (EBL). Second, mildly radiative-cooled shocks may lead to a harder CR spectrum with spectral indices of $1.5\lesssim s\lesssim2.0$. Our study suggests that halo mergers, a fraction of which may also induce starbursts in the merged galaxies, can be promising neutrino emitters without violating the existing $Fermi$ $γ$-ray constraints on the non-blazar component of the extragalactic $γ$-ray background.
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