The LOFAR view of intergalactic magnetic fields with giant radio galaxies

Abstract

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.

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

“…The exception is the FR I radio galaxy J0621-5647 in the far south, at the same z as the cluster pair. Polarisation measurements of these GRGs could be interesting for rotation-measure studies of the magnetic fields in the double cluster region (Pratley et al 2013;Stuardi et al 2020). Finally, there are some other notable radio galaxies that show signs of interaction with the ICM of A3391-A3395, such as the wide-angle tail source F2 as well as the radio galaxy F1.…”

Simulations and observational tests of primordial magnetic fields from Cosmic Microwave Background constraints

“…completely depolarized), but would be present in data at a higher frequency, such as at 1.4 GHz in the NVSS (Vernstrom et al 2019). Recently, a depolarization study of LOFAR polarized sources found that they have a typical RM dispersion within the beam of <0.3 rad m −2 (Stuardi et al 2020). Therefore, any radio galaxy that is embedded in an environment with RM fluctuations larger than this on scales smaller than 20 arcsec will not be present in the LOFAR data, but can be detected at 1.4 GHz (for RM dispersions up to a few 10s of rad m −2 ).…”

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

“…Gaensler et al 2004) and associated advanced analysis techniques (e.g. Vernstrom et al 2019;O'Sullivan et al 2020;Stuardi et al 2020) might effectively probe such material. The thermal electron density drops rapidly with cluster-centric radius in typical ICM models, and the efficiency of Bremsstrahlung as a tracer of this gas drops more precipitously, since its emissivity is proportional to n 2 e .…”