A low-frequency study of linear polarization in radio galaxies
A low-frequency study of linear polarization in radio galaxies
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DOI：
10.1093/mnras/staa3980
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发表时间：
2020-12
影响因子：
4.8
通讯作者：
V. Mahatma;M. Hardcastle;J. Harwood;Shane O'Sullivan;G. Heald;C. Horellou;Daniel J. B. Smith
中科院分区：
文献类型：
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作者：
V. Mahatma;M. Hardcastle;J. Harwood;Shane O'Sullivan;G. Heald;C. Horellou;Daniel J. B. Smith
射电星系是线性偏振的-这是一个重要的性质，使我们能够推断源及其环境的磁场的性质。然而，在低频率下，法拉第旋转实质上使辐射去极化，这意味着在低频率下已知的极化射电星系相对较少。使用LOFAR两米巡天，在150兆赫和20角秒的分辨率，我们选择了342个射电星系亮度超过50毫焦耳和大于100角秒的角大小，其中67个是偏振的（18%的检测分数）。这些主要是Fanaroff-Riley II型源。检测分数增加总通量密度，并超过50%的来源，亮度超过1焦耳。我们在我们的样品中检测到的低频阵列射线（LOFAR）的源也检测到在1400 MHz的NRAO VLA巡天，并发现我们的选择偏向明亮的射电星系驱动器的趋势源去极化之间的1400和150 MHz的频率范围内比那些保持极化在150 MHz的频谱更平坦。通过比较观测到的旋转措施与分析模型，我们发现，我们优先敏感的低质量环境中的源。我们还推断，与一个偏振热点的源是由一个小角度的视线倾斜，而在两个瓣热点的源位于天空的平面。我们的结论是，在射电星系的低频偏振环境，流量密度和喷流方向的组合有关。
Radio galaxies are linearly polarized – an important property that allows us to infer the properties of the magnetic field of the source and its environment. However, at low frequencies, Faraday rotation substantially depolarizes the emission, meaning that comparatively few polarized radio galaxies are known at low frequencies. Using the LOFAR Two-metre Sky Survey at 150 MHz and at a resolution of 20 arcsec, we select 342 radio galaxies brighter than 50 mJy and larger than 100 arcsec in angular size, of which 67 are polarized (18 per cent detection fraction). These are predominantly Fanaroff–Riley type II sources. The detection fraction increases with total flux density, and exceeds 50 per cent for sources brighter than 1 Jy. We compare the sources in our sample detected by the LOw Frequency ARray (LOFAR) to those also detected in NRAO VLA Sky Survey at 1400 MHz, and find that our selection bias towards bright radio galaxies drives a tendency for sources depolarized between 1400 and 150 MHz to have flatter spectra over that frequency range than those that remain polarized at 150 MHz. By comparing observed rotation measures with an analytic model, we find that we are preferentially sensitive to sources in low-mass environments. We also infer that sources with one polarized hotspot are inclined by a small angle to the line of sight, while sources with hotspots in both lobes lie in the plane of the sky. We conclude that low-frequency polarization in radio galaxies is related to a combination of environment, flux density, and jet orientation.