Investigation of the Radial Profile of Galactic Magnetic Fields using Rotation Measure of Background Quasars

Abstract

Probing magnetic fields in high-redshift galactic systems is crucial to investigate galactic dynamics and evolution. Utilizing the rotation measure of the background quasars, we have developed a radial profile of the magnetic field in a typical high-$z$ galaxy. We have compiled a catalog of 59 confirmed quasar sightlines, having one intervening Mg \rom{2} absorber in the redshift range $0.372\le z_{\text{abs}}\le 0.8$. The presence of the foreground galaxy is ensured by comparing the photometric and spectroscopic redshifts within $3{\sigma }_{z-\text{photo}}$ and visual checks. These quasar line-of-sights (LoS) pass through various impact parameters (D) up to $160$ kpc, covering the circumgalactic medium of a typical Milky-Way type galaxy. Utilizing the residual rotation measure (RRM) of these sightlines, we estimated the excess in RRM dispersion, ${\sigma }_{\text{ex}}^{\text{RRM}}$. We found ${\sigma }_{\text{ex}}^{\text{RRM}}$ decreases with increasing D. We translated ${\sigma }_{\text{ex}}^{\text{RRM}}$ to average LoS magnetic field strength, $⟨B_{\Vert }⟩$ by considering a typical electron column density. Consequently, the decreasing trend is sustained in the magnetic field. In particular for sightlines with $\text{D}\le 50$ kpc and $\text{D}>50$ kpc, $⟨B_{\Vert }⟩$ is found to be $2.39\pm 0.7\mu$G and $1.67\pm 0.38\mu$G, respectively. This suggests a clear indication of varying magnetic field from the disk to the circumgalactic medium. This work provides a methodology that, when applied to ongoing and future radio polarisation surveys such as LOFAR and SKA, promises to significantly enhance our understanding of magnetic field mapping in galactic systems.

arXiv commentPublished in JCAP

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