2020ApJ...890..132M

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 the mean (σ_rrm^md), and found it to be 17.1 ± 0.7 rad m^–2 for 352 sightlines having Mg II intervening absorbers in comparison to its value of 15.1 ± 0.6 rad m^–2 for 780 sightlines without such absorbers, resulting in an excess broadening (σ_rrm^ex) of 8.0 ± 1.9 rad m^–2 among these two subsamples. This value of σ_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 ∼1.3 ± 0.3 µG 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 the local universe. A similar analysis on subsample split based on the radio spectral index, α (with F_ν ∝ ν^α), for flat (α >= -0.3; 315 sources) and steep (α <= -0.7; 476 sources) spectrum sources shows a significant σ_rrm^ex (at 3.5σ level) for the former and absent in the latter. An anticorrelation found between the σ_rrm^md and percentage polarization (p) with a similar Pearson correlation of -0.62 and -0.87 for subsamples with and without Mg II, respectively, suggests the main contribution for decrements in the p value to be intrinsic to the local environment of quasars.