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arXiv 0812.0346astro-ph
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星暴星系NGC 253 I附近的宇宙射线和磁场。宇宙射线的分布和传输

Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 I. The distribution and transport of cosmic rays

V. Heesen, R. Beck, M. Krause, R. -J. Dettmar

摘要:

本文用局部CR体速度研究了NGC 253中从盘到晕的扩散和对流CR输运。CR运输和银河风之间的连接进行了概述。我们观察到NGC 253的VLA在λ 6.2厘米的马赛克与15点。使用100米Effelsberg望远镜的观测结果填补了VLA镶嵌图中缺失的零间距通量密度。我们还获得了一个新的λ 3.6厘米的地图从Effelsberg的观测和复制VLA地图在λ 20厘米和λ 90厘米。在λ 6.2cm处发现了一个薄射电盘和一个厚射电盘,其指数标度高度分别为0.3kpc和1.7kpc。在7微G和18微G之间,磁盘中的均分总磁场强度非常高。我们利用谱指数垂直剖面中宇宙线电子(CREs)的谱老化来确定全球CR整体速度的下限为(170+/-70)km/s。从哑铃状晕中可以看出,标度高度与CRE寿命之间存在线性关系,这要求在东北晕中有一个整体速度为(300+/-30)km/s的垂直CR输运,类似于280 km/s的逃逸速度。这表明在NGC 253中存在“盘风”。在西南晕中,输运主要是扩散性的,扩散系数为(2.0+/-0.2)10 E29 cm ^2 s^-1。在东北晕,CR传输是对流的,比西南晕更有效。发光物质是由盘风输送的,这可以解释两个晕部分中不同数量的面外HI,Halpha和软X射线发射。未来的低频射电观测将提供数据来分析银河风的垂直速度分布。

英文摘要:

The diffusive and convective CR transport in NGC 253 from the disk into the halo is investigated using the local CR bulk speed. The connection between the CR transport and the galactic wind is outlined. We observed NGC 253 with the VLA at lambda 6.2 cm in a mosaic with 15 pointings. The missing zero-spacing flux density of the VLA mosaic was filled in using observations with the 100-m Effelsberg telescope. We also obtained a new lambda 3.6 cm map from Effelsberg observations and reproduced VLA maps at lambda 20 cm and lambda 90 cm. We find a thin and a thick radio disk with exponential scaleheights of 0.3 kpc and 1.7 kpc at lambda 6.2 cm. The equipartition total magnetic field strength between 7 micro G and 18 micro G in the disk is remarkably high. We use the spectral aging of the cosmic ray electrons (CREs) seen in the vertical profiles of the spectral index to determine a lower limit for the global CR bulk speed as (170+/-70) km/s. The linear correlation between the scaleheights and the CRE lifetimes, as evident from the dumbbell shaped halo, requires a vertical CR transport with a bulk speed of (300+/-30) km/s in the northeastern halo, similar to the escape velocity of 280 km/s. This shows the presence of a "disk wind" in NGC253. In the southwestern halo, the transport is mainly diffusive with a diffusion coefficient of (2.0+/-0.2) 10E29 cm^2 s^-1. In the northeastern halo, the CR transport is convective and more efficient than in the southwestern halo. The luminous material is transported by the disk wind, which can explain the different amounts of extra-planar HI, Halpha, and soft X-ray emission in the two halo parts. Future low-frequency radio observations will provide the data to analyze the vertical velocity profile of galactic winds.

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