Detection of magnetic fields in the circumgalactic medium of nearby galaxies using Faraday rotation

Abstract

Context. The existence of magnetic fields in the circumgalactic medium (CGM) is largely unconstrained. Their detection is important as magnetic fields can have a significant impact on the evolution of the CGM and, in turn, the fields can serve as tracers for dynamical processes in the CGM. Aims. With Faraday rotation of polarised background sources, we aim to detect a possible excess of the rotation measure in the surrounding area of nearby galaxies. Methods. We use 2,461 residual rotation measures (RRMs) observed with the LOw Frequency ARray (LOFAR), where the foreground contribution from the Milky Way is subtracted. The RRMs are then studied around a subset of 183 nearby galaxies that was selected by apparent $B$-band magnitude. Results. We find that, in general, the RRMs show no significant excess for small impact parameters (i.e. the perpendicular distance to the line of sight). However, if we only consider galaxies at higher inclination angles and sight lines that pass close to the minor axis of the galaxies, we find significant excess at impact parameters of less than 100 kpc. The excess in |RRM| is 3.7 $\mathrm{rad}{\mathrm{m}}^{-2}$ with an uncertainty between $\pm 0.9\mathrm{rad}{\mathrm{m}}^{-2}$ and $\pm 1.3\mathrm{rad}{\mathrm{m}}^{-2}$ depending on the statistical properties of the background (2.8$\sigma$-4.1$\sigma$). With electron densities of ~$10^{-4}\mathrm{c}{\mathrm{m}}^{-3}$ this suggests magnetic field strengths of a few tenths of a micro Gauss. Conclusions. Our results suggest a slow decrease of the magnetic field strength with distance from the galactic disc such as expected if the CGM is magnetised by galactic winds and outflows.

arXiv comment5 pages, 4 figures, accepted as Letter to Astronomy and Astrophysics

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