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

Sunil Malik; Hum Chand; T. R. Seshadri

doi:10.3847/1538-4357/ab6bd5

paper

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

arXiv:1710.10396 · doi:10.3847/1538-4357/ab6bd5

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 mean ( $σ_{r r m}^{m d}$ ), and found it to be 17.1 $\pm 0.7$ rad m $^{- 2}$ for 352 sightlines having Mg II intervening absorbers in comparison to its value of $15.1 \pm 0.6$ rad m $^{- 2}$ for 780 sightlines without such absorbers, resulting in an excess broadening ( $σ_{r r m}^{e x}$ ) of $8.0 \pm 1.9$ rad m $^{- 2}$ among these two subsamples. This value of $σ_{r r m}^{e x}$ , 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 $\sim 1.3 \pm 0.3 μ G$ 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 local universe. A similar analysis on subsample split based on the radio spectral index, $α$ , (with $F_{ν} \propto ν^{α}$ ) for flat ( $α$ $\geq - 0.3$ ; 315 sources) and steep ( $α$ $\leq - 0.7$ ; 476 sources) spectrum sources shows a significant $σ_{r r m}^{e x}$ (at 3.5 $σ$ level) for the former and absent in latter. An anti-correlation found between the $σ_{r r m}^{m d}$ 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.

11 pages, 8 figures, 4 tables

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

Abstract

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