We have investigated the dependance of the residual rotation measure (RRM) on the
intervening Mg ii absorber at cosmic distances, by using a large sample of 340 quasars
with 518 Mg ii absorbers and 565 quasars without Mg ii absorbers from SDSS and most
updated quasar catalog for RM around 21cm wavelength. After discounting the contribution
of foreground galactic rotation measure (GRM) of milky way, the residual rotation measure
(RRM) of these quasars is used to calculate the excess extragalactic contribution in the
standard deviation of the distribution of observed RRM. The excess standard deviation of
the sample with intervening Mg ii absorber of about 47.53 ± 0.52 rad m−2 is found in
comparison of the sample without Mg ii absorber. Our results suggest that intervening
absorbers could contribute in enhancement of RRM at around 21cm wavelength, with a
sample which is about twice the largest sample used in literature till now.
We have also obtained the evolution of RRM with redshift, which gets saturated at higher
z  3 with no Mg ii absorber. It supports the conclusion that magnetic field strength traced
by RRM for high redshift galaxies is atleast comparable to the current epoch. The evolution
of RRM with redshift also shows that dispersion in RRM is higher for the sample with
Mg ii absorber as compare to the sightlines without Mg ii absorber.