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