Role of Intervening Mg ii Absorbers on the Rotation Measure and Fractional Polarization of the Background Quasars
Sunil Malik, Hum Chand, T. R. Seshadri
Abstract
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 the mean ( ), 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 ( ) of 8.0 ± 1.9 rad m −2 among these two subsamples. This value of , 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 μ 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 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 (at 3.5 σ level) for the former and absent in the latter. An anticorrelation found between the 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.