The LOFAR view of intergalactic magnetic fields with giant radio galaxies
The LOFAR view of intergalactic magnetic fields with giant radio galaxies
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DOI：
10.1051/0004-6361/202037635
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发表时间：
2020-04
影响因子：
6.5
通讯作者：
C. Stuardi;S. O’Sullivan;A. Bonafede;M. Brüggen;P. Dabhade;C. Horellou;R. Morganti;E. Carretti
中科院分区：
文献类型：
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作者：
C. Stuardi;S. O’Sullivan;A. Bonafede;M. Brüggen;P. Dabhade;C. Horellou;R. Morganti;E. Carretti
上下文。巨型射电星系（GRGs）是物理上的大型射电源，它远远超出了宿主星系的环境。它们的极化特性受到在百万秒差距尺度上渗透到星系间介质的约束不佳的磁场的影响。LOFAR现在可以对这类射电源进行低频（< 200 MHz）极化研究。目标。在这里，我们研究了在LOFAR两米巡天中探测到的一个GRGs目录的偏振特性和法拉第旋转测量（RM）。这是第一次对根据物理大小选择的大量射电星系样本进行低频极化研究。我们探索了它们低密度环境的磁离子特性，并利用它们射电叶的法拉第旋转特性探测了星系间磁场。LOFAR是这种分析的关键仪器，因为它可以探测少量的法拉第色散（< 1 rad m−2），这与弱磁场和低热气体密度有关。方法。我们在120 ~ 168 MHz频段使用RM合成来搜索极化发射，并推导出每个检测到的源分量的RM和分数极化。我们利用NRAO VLA巡天的图像研究了1.4 GHz和144 MHz之间的去极化。我们研究了探测率、波瓣间RM差和去极化在不同参数下的相关性：光源的角和线尺寸以及距离最近的前景星系团的投影距离。在我们的样本中，我们还包括了3C 236，它是已知最大的射电星系之一。结果。从240个GRGs样品中，我们检测到37个偏振源，它们的总通量密度都在56 mJy以上。我们检测到叶瓣之间的显著RM差异，这在千兆赫频率下是无法达到的，中值为1 rad m−2。在1.4 GHz和144 MHz频段，检测到的GRGs的分数极化与少量的法拉第去极化（法拉第色散< 0.3 rad m−2）一致。我们的分析表明，这些叶片正在扩展到一个低密度（< 10−5 cm−3）的局部环境，该环境被弱磁场（< 0.1 μG）所渗透，其波动尺度为3−25 kpc。前景星系团的存在对偏振探测率的影响高达2R500。总的来说，这项工作证明了LOFAR量化这些GRGs存在的稀薄环境的能力，并强调它们是一个很好的统计样本，可以用作星系间介质和银河系磁场的高精度探测器。
Context. Giant radio galaxies (GRGs) are physically large radio sources that extend well beyond their host galaxy environment. Their polarization properties are affected by the poorly constrained magnetic field that permeates the intergalactic medium on megaparsec scales. A low frequency (< 200 MHz) polarization study of this class of radio sources is now possible with LOFAR. Aims. Here we investigate the polarization properties and Faraday rotation measure (RM) of a catalog of GRGs detected in the LOFAR Two-meter Sky Survey. This is the first low frequency polarization study of a large sample of radio galaxies that were selected on their physical size. We explore the magneto-ionic properties of their under-dense environment and probe intergalactic magnetic fields using the Faraday rotation properties of their radio lobes. LOFAR is a key instrument for this kind of analysis because it can probe small amounts of Faraday dispersion (< 1 rad m−2), which are associated with weak magnetic fields and low thermal gas densities. Methods. We used RM synthesis in the 120−168 MHz band to search for polarized emission and to derive the RM and fractional polarization of each detected source component. We study the depolarization between 1.4 GHz and 144 MHz using images from the NRAO VLA Sky Survey. We investigate the correlation of the detection rate, the RM difference between the lobes, and the depolarization with different parameters as follows: the angular and linear size of the sources and the projected distance from the closest foreground galaxy cluster. In our sample, we also included 3C 236, which is one of the largest radio galaxies known. Results. From a sample of 240 GRGs, we detected 37 sources in polarization, all of which have a total flux density above 56 mJy. We detected significant RM differences between the lobes, which would be inaccessible at gigahertz frequencies, with a median value of ∼1 rad m−2. The fractional polarization of the detected GRGs at 1.4 GHz and 144 MHz is consistent with a small amount of Faraday depolarization (a Faraday dispersion < 0.3 rad m−2). Our analysis shows that the lobes are expanding into a low-density (< 10−5 cm−3) local environment that is permeated by weak magnetic fields (< 0.1 μG) with fluctuations on scales of 3−25 kpc. The presence of foreground galaxy clusters appears to influence the polarization detection rate up to 2R500. In general, this work demonstrates the ability of LOFAR to quantify the rarefied environments in which these GRGs exist and highlights them as an excellent statistical sample to use as high precision probes of magnetic fields in the intergalactic medium and the Milky Way.