A preliminary image of the nearby Coma cluster as seen by LOFAR at 140 MHz. LOFAR recovers the large-scale radio halo and relic in this cluster, connected by a bridge of diffuse radio emission, as well as many compact radio sources. The overall extent of the radio emission is around 10 million light years.
The detection and investigation of magnetic fields in galaxy clusters, filaments and voids of the cosmic web is crucial to shed light on the history of cosmological magnetic fields. To date, magnetic fields have been firmly detected in galaxy clusters, while beyond the galaxy cluster periphery indication of their presence is finally coming from the new generation of radio telescopes.
Diffuse large scale synchrotron sources and the Faraday rotation effect on the signal of background and embedded radio sources are powerful tools to study magnetization of the large-scale structure of the Universe and can be very well studied with instruments such as LOFAR. Low-frequency observations are ideal for investigating diffuse synchrotron emission in galaxy clusters and beyond, due to their steep spectrum (α >1, Sν ∝ ν−α). Large-bandwidth observations at low frequencies are provide powerful constraints on Faraday depth values, providing uncertainties down to ~0.1 rad/m2 that allow us to investigate magnetic field properties both through detailed observations of individual targets as well as on statistical samples.
In the context of the activities of the working group, we are addressing the detection and investigation of large-scale magnetic fields by applying the two approaches described above and by developing sophisticated techniques of analysis. These activities are done in collaboration with other working groups, as the Radio Galaxy Working Group in the MKSP and the Cluster Working Group in the SKSP.
The synchrotron emission from the cosmic web is likely too faint and or diffuse to be seen in images. However, this emission should be correlated with the number density of galaxies from infrared and optical surveys. This is shown in the Figure, with the top showing simulated synchrotron emission from an MHD simulation (credit K. Dolag) and the bottom showing the number density from the 2MASS spectroscopic survey. Computing the cross correlation function of deep LOFAR images with galaxy number densities maps should allow us to detect or constrain the flux density of this emission and the corresponding magnetic fields strengths of the Intergalactic magnetic fields. This technique was demonstrated using low frequency MWA data and WISE number densities (Vernstrom et al. 2017), with one of the cross correlation functions shown in the image to the right.
Recently, Vacca et al. (2018) documented detection of diffuse radio emission potentially connected to an overdensity at redshift z~0.1, traced by nine massive galaxy clusters. They used a combination of 1.4 GHz data from the Sardinia Radio Telescope and data from the NRAO VLA Sky Survey (NVSS). These diffuse sources exhibit larger size and fainter X-ray emission than known diffuse large-scale synchrotron cluster sources at a given radio power.
They could represent the tip of the iceberg of a class of diffuse large-scale synchrotron sources associated with the filaments of the cosmic web. Since these sources are expected to have a steep spectrum, we observed the same region of the sky with Lofar at 140 MHz. The image shows the SRT and SRT+NVSS emission at 1.4 GHz. Circles indicate the areas observed by Lofar at 120 MHz.
Coma is a prominent nearby galaxy cluster, known to host diffuse radio emission in the form of a radio halo and radio relic, connected by a bridge of diffuse radio emission. The Figure to the righ presents a preliminary image produced from LOFAR HBA observations at 140 MHz and a bandwidth of 16 MHz. The resolution is approximately 25 arcsec (rms ~0.6 mJy/beam, 1/3 of data processed). The LOFAR data can be used to investigate the properties of the magnetic fields in this region of the sky by analysing the total intensity and polarization properties of diffuse and discrete sources in the field.