Colloquium - Linear polarization in radio galaxies at low frequencies

© public

Early radio and optical observations of radio galaxies inferred that the mechanism by which they predominantly radiate is the synchrotron process, and that the radio emission we observe is polarized. Polarization observations are extremely useful, as they directly trace the magnetic field structure in radio galaxies, as well as their large-scale environment, which can otherwise only be directly obtained from sensitive X-ray observations. In the past, polarization observations of radio galaxies, particularly at low frequencies, have been severely limited. The nature of Faraday rotation means that most radio sources become depolarized at low frequencies, and low frequency samples of the polarized radio galaxy population are missing. LOFAR is now beginning to unveil a large number of polarized radio-loud quasars and radio galaxies (Van Eck+, O'Sullivan+).

In this talk I present the largest systematically-selected sample of polarized radio galaxies at 150 MHz. We use the LOFAR Two Meter Sky Survey (LoTSS) to search for polarization amongst 380 of the brightest and largest radio galaxies, obtaining a detection fraction of 20%. We analyse their polarized emission, fractional polarization, Faraday rotation and host galaxy properties in a statistical sense. We also attempt to infer the distribution of radio galaxy environments expected of our polarized sample. This study will be useful for interpreting the polarization of sources in upcoming radio surveys at higher frequencies (SKA, VLASS), for which polarization studies are a primary driver.

The image shows 20" LOFAR linear polarization maps of radio galaxies selected from LoTSS DR1. Coloured pixels represent linearly polarized intensity, with contours showing the 6" total intensity.