Dependence of residual rotation measure on intervening Mg ii absorbers at cosmic distances

We investigate the dependence of residual rotation measure (RRM) on intervening absorption systems at cosmic distances by using a large sample of 539 Sloan Digital Sky survey quasars in conjunction with the available rotation measure catalogue at around 21 cm wavelength. We found an excess extragalactic contribution in the standard deviation of an observed RRM (σrrm) of about 8.11 ± 4.83 rad m−2 in our sample with an intervening Mg ii absorber as compared to the sample without an Mg ii absorber. Our results suggest that intervening absorbers could contribute to the enhancement of RRM at around 21 cm wavelength, as was found earlier for rotation measure measurements at around 6 cm wavelength.