A Global Probe of Cosmic Magnetic Fields to High Redshifts

TL;DR

The study leverages a large high-latitude RM dataset extending to $z\sim 3.7$ to probe the evolution of cosmic magnetic fields via the distribution of extragalactic RM after foreground subtraction. By decomposing RM into Galactic, intervening, intrinsic, and noise components and applying a Bayesian Gaussian Process foreground model, the authors demonstrate a redshift-driven broadening of the RM distribution, with a notable decline in sightlines having $|\mathrm{RRM}|\lesssim 20$ rad m$^{-2}$ beyond $z\sim 1$. They develop an intervenor model based on MgII absorber statistics, yielding best-fit magnetized-cloud parameters ($\sigma_{cloud} \approx 115$ rad m$^{-2}$, $\Gamma_{intr}/2 \approx 7$ rad m$^{-2}$) that reproduce the observed trends up to $z\sim 2$, and infer that early galaxies likely hosted $\mu$G-level magnetic fields. While the data disfavors a purely coeval, pervasive IGM field within current sensitivity, it establishes a framework for constraining primordial or intergalactic magnetism with future deeper RM surveys to $z\sim 4$–$5$.

Abstract

Faraday rotation (RM) probes of magnetic fields in the universe are sensitive to cosmological and evolutionary effects as $z$ increases beyond $\sim $1 because of the scalings of electron density and magnetic fields, and the growth in the number of expected intersections with galaxy-scale intervenors, $d$N/$dz$. In this new global analysis of an unprecedented large sample of RM's of high latitude quasars extending out to $z\sim $3.7 we find that the distribution of RM broadens with redshift in the 20 $-$ 80 rad m$^{-2}$ range range, despite the (1 +$z$)$^{-2}$ wavelength dilution expected in the observed Faraday rotation. Our results indicate that the Universe becomes increasingly ``Faraday-opaque'' to sources beyond $z \sim$ 2, that is, as $z$ increases progressively fewer sources are found with a ``small'' RM in the observer's frame. This is in contrast to sources at $z \la$1. They suggest that the environments of galaxies were significantly magnetized at high redshifts, with magnetic field strengths that were at least as strong within a few Gyr of the Big Bang as at the current epoch. We separately investigate a simple unevolving toy model in which the RM is produced by MgII absorber systems, and find that it can approximately reproduce the observed trend with redshift. An additional possibility is that the intrinsic RM associated with the radio sources was much higher in the past, and we show that this is not a trivial consequence of the higher radio luminosities of the high redshift sources.

Figures (7)

• Figure 1: A realization of a process convolution model on the unit circle. The left panel shows knot locations and values (positive and negative), along with a smoothing kernel. The right panel shows the resulting Gaussian process realization obtained by a convolution of the knot values with the smoothing kernel.
• Figure 3: $\left|\rm{RRM}\right|$ histograms of 268 lines of sight for different redshift bins having approximately equal numbers of lines of sight per bin. Redshift increases from the upper left to the lower right panel. The distribution in the highest redshift bin ($z \ga$ 1.8) is characterized by a significantly decreased "peak" near $|$RRM$|$ = 0, and a corresponding significant broadening of the "low-$|$RRM$|$ " peak to higher $|$RRM's$|$.
• Figure 4: Fraction of lines of sight $\rm{f_{i}}$ below a threshold $\rm{RRM_{i}}$ as a function of redshift. Circles, triangles and squares show $\rm{f_{20}}$, $\rm{f_{40}}$ and $\rm{f_{100}}$ respectively. Errors are calculated by randomly drawing lines of sight and calculating the r.m.s. of the resulting $f_{i}$ distributions.
• Figure 5: Left panel: Significance level (SL) of the KS test that the distributions of the RRM's below and above $z_{b}$ are not drawn from the same parent distribution. The black solid line is with no $\left|\rm{RRM} \right|$ cut and the blue line is for lines of sight with $\left|\rm{RRM} \right| < 200$$\rm{rad/m^{2}}$. The dashed dotted lines give the fraction of lines of sight which are below $z_{b}$ (right-hand axis). Right panel: The RRM value at which the normalized cumulative distributions, N(RRM) from the KS-test most differ, as a function of $z_{b}$.
• Figure 6: A comparison of the normalized cumulative counts of N$(< |\rm{RRM}|)$ vs. $|\rm{RRM}|$ shown separately for sources having z $<$ 1 (upper curve) and z $>$ 1 (lower curve). The clear separation between these two curves shows that excess RRM's introduced in the higher redshift subset are typically in the range $\left|\rm{RRM} \right|$$\sim$ 20 to $\sim$ 80 rad m$^{-2}$ in the observer's frame.