星系晕中对流和扩散cosmic ray输送
Advective and diffusive cosmic ray transport in galactic haloes
摘要：
我们提出了一维宇宙线输运模型，数值求解了电子的纯平流和扩散方程，计算了同步辐射发射谱。我们发现，指数晕磁场分布平流导致近似指数无线电连续强度分布，而扩散导致的配置文件，可以更好地近似高斯函数。因此，平流的垂直无线电频谱剖面近似为线性，而扩散的垂直无线电频谱剖面则为“抛物线”形状。我们比较我们的模型与深ATCA观测的两个边缘上的星系，NGC 7090和7462，在$λλ$ 22和6厘米。结果表明，NGC7090的宇宙线输运是以$V=150^{+80}_{-30}~\rm km\,s^{-1}$为主的平流输运，而NGC7462的宇宙线输运是以$D=3.0\pm 1.0 \times 10^{28}E_{\rm GeV}^{0.5}~\rm cm^2\,s^{-1}$为主的扩散输运。NGC 7090有一个薄的和厚的射电盘，其磁场标度高度分别为$h_{\rm B1}=0.8\pm 0.1$ kpc和$h_{\rm B2}=4.7\pm 1.0$ kpc。NGC 7462只有一个$h_{\rm B2}=3.8\pm 1.0$ kpc厚的射电盘。在这两个星系中，磁场标度高度明显小于能量均分的估计值。一个不可忽略的宇宙射线电子部分可以从NGC 7090逃逸，所以这个星系不是一个电子量热计。
英文摘要：
We present 1D cosmic ray transport models, numerically solving equations of pure advection and diffusion for the electrons and calculating synchrotron emission spectra. We find that for exponential halo magnetic field distributions advection leads to approximately exponential radio continuum intensity profiles, whereas diffusion leads to profiles that can be better approximated by a Gaussian function. Accordingly, the vertical radio spectral profiles for advection are approximately linear, whereas for diffusion they are of `parabolic' shape. We compare our models with deep ATCA observations of two edge-on galaxies, NGC 7090 and 7462, at $λλ$ 22 and 6 cm. Our result is that the cosmic ray transport in NGC 7090 is advection dominated with $V=150^{+80}_{-30}~\rm km\,s^{-1}$, and that the one in NGC 7462 is diffusion dominated with $D=3.0\pm 1.0 \times 10^{28}E_{\rm GeV}^{0.5}~\rm cm^2\,s^{-1}$. NGC 7090 has both a thin and thick radio disc with respective magnetic field scale heights of $h_{\rm B1}=0.8\pm 0.1$ kpc and $h_{\rm B2}=4.7\pm 1.0$ kpc. NGC 7462 has only a thick radio disc with $h_{\rm B2}=3.8\pm 1.0$ kpc. In both galaxies, the magnetic field scale heights are significantly smaller than what estimates from energy equipartition would suggest. A non-negligible fraction of cosmic ray electrons can escape from NGC 7090, so that this galaxy is not an electron calorimeter.
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