Role of intervening Mg II absorbers on the rotation measure and fractional polarisation of the background quasars

Abstract

We probed the magnetic fields in high-redshift galaxies using excess extragalactic contribution to residual rotation measure (RRM) for quasar sightlines with intervening Mg II absorbers. Based on a large sample of 1132 quasars, we have computed RRM distributions broadening using median absolute deviation from mean ($\sigma^{md}_{rrm}$), and found it to be 17.1$\pm0.7$ rad m$^{-2}$ for 352 sightlines having Mg II intervening absorbers in comparison to its value of $15.1\pm0.6$ rad m$^{-2}$ for 780 sightlines without such absorbers, resulting in an excess broadening ($\sigma_{rrm}^{ex}$) of $8.0\pm1.9$ rad m$^{-2}$ among these two subsamples. This value of $\sigma_{rrm}^{ex}$, has allowed us to constrain the average strength of magnetic field (rest frame) in high redshift galaxies responsible for these Mg II absorbers, to be $\sim 1.3\pm0.3 \mu G$ at a median redshift of 0.92. This estimate of magnetic field is consistent with the reported estimate in earlier studies based on radio-infrared correlation and energy equipartition for galaxies in local universe. A similar analysis on subsample split based on the radio spectral index, $\alpha$, (with $F_{\nu}\propto \nu^{\alpha}$) for flat ($\alpha$ $\geq -0.3$; 315 sources) and steep ($\alpha$ $\leq -0.7$; 476 sources) spectrum sources shows a significant $\sigma_{rrm}^{ex}$ (at 3.5$\sigma$ level) for the former and absent in latter. An anti-correlation found between the $\sigma^{md}_{rrm}$ and percentage polarisation ($p$) with similar Pearson correlation of $-0.62$ and $-0.87$ for subsample with and without Mg II, respectively, suggests main contribution for decrements in the $p$ value to be intrinsic to the local environment of quasars.