Strong magnetic fields in normal galaxies at high redshifts

TL;DR

High-resolution spectra are reported that demonstrate that the quasars with strong Mg ii absorption lines are unambiguously associated with larger rotation measures.

Abstract

The origin and growth of magnetic fields in galaxies is still something of an enigma. It is generally assumed that seed fields are amplified over time through the dynamo effect, but there are few constraints on the timescale. It has recently been demonstrated that field strengths as traced by rotation measures of distant quasars are comparable to those seen today, but it was unclear whether the high fields were in the exotic environments of the quasars themselves or distributed along the line of sight. Here we demonstrate that the quasars with strong MgII absorption lines are unambiguously associated with larger rotation measures. Since MgII absorption occurs in the haloes of normal galaxies along the sightline to the quasars, this association requires that organized fields of surprisingly high strength are associated with normal galaxies when the Universe was only about one-third of its present age.

Figures (2)

• Figure 1: Distributions of RM for different numbers of strong MgII absorption
• Figure 2: Cumulative distributions of Rotation Measures of sightlines with and without strong MgII absorption line systems. The 31 sightlines with one or two strong MgII systems correspond to the blue solid line, those with two strong MgII systems to the black dashed line. The 40 sightlines without strong MgII absorption lines are represented by the red dashed line. For clarity, only the range$0<\mathrm{RM}<200 \mathrm{rad} / \mathrm{m}^{2}$ is shown. A typical RM measurement error is around $3 \mathrm{rad} \mathrm{m}^{-2}$. A KS-test indicates that the distributions of RM for $\mathrm{N}_{\text{MgII }}=0$ and $\mathrm{N}_{\text{MgII }}>0$ are different at the 92.2 $\%\left(96.1 \%\right.$ for $\left.\mathrm{Z}_{\mathrm{qso}}>1\right)$ and those for $\mathrm{N}_{\mathrm{MgII}}=0$ and $\mathrm{N}_{\mathrm{MgII}}=2$ at the $99.93 \%$ significance level.