An Intracluster Investigation via Radio Rotation

Title: Evidence for strong intracluster magnetic fields in the early Universe

Authors: J. Xu and J. L. Han

First Author’s Institution: National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, China

Status: Accepted by ApJ [open access on arXiv]

Mysteries of the magnetic macrocosm

Magnetic fields are everywhere, from the vast, pristine emptiness of cosmic voids to the dense, galaxy-packed environments of massive clusters. Wherever we find plasma – the hot, ionized fluid making up 99.9% of the Universe’s visible matter – we find magnetic fields shaping and stirring said plasma. Needless to say, magnetic fields have their fingers (or, rather, their field lines) in many pies. Yet, for a wide range of astrophysical situations, the question still remains: where did these fields come from?

Galaxy clusters – associations of hundreds to thousands of galaxies held together by the glue of gravity – are no exception to this magnetic mystery. Today’s paper seeks to understand the origin of magnetic fields in the intracluster medium (or ICM), the ultra-hot plasma permeating the space between cluster-bound galaxies. At surface level, knowledge of the magnetic fields in the ICM is necessary for understanding the rich spectrum of radiation emitted by galaxy clusters (see, for instance, these three Astrobites). On a grander scale, however, these clusters – as the largest gravitationally-bound bodies in the Universe – can also provide key insight into the history of our cosmos. By tracing the growth of intracluster fields back through cosmic time, we can probe how magnetic fields influenced the formation of structure in the infant Universe and catch a glimpse of the earliest magnetic fields in existence. This is precisely what today’s authors set out to do.

Faraday forecasts of faraway fields

So, how does one study magnetic fields that are millions to billions of lightyears from Earth? Today’s authors leverage the power of Faraday rotation: when a polarized light wave passes through a magnetic field, the wave is rotated through an angle that depends on the strength of the field (as illustrated in this cartoon). Therefore, by observing the change in the polarization angle of incoming light and calculating the so-called “rotation measure” (or RM), one can deduce the strength of the magnetic field along the light’s path. This technique is invaluable in radio astronomy, and has been used extensively to study the magnetic backdrop of our Universe.

If we’re going to be using Faraday rotation to explore intracluster magnetic fields, all we need now is some radiating object to shine light through a galaxy cluster and into our telescopes. There’s one slight complication, though: the incoming light is sensitive to magnetic fields along its entire path of propagation – if we’re looking at light from a distant galaxy cluster, the wave will be rotated not only by the intracluster fields, but also by intergalactic fields between the cluster and the Milky Way and by Galactic fields within the Milky Way. How, then, do we isolate the rotation due solely to intracluster fields?

Figure 1: Composite photo of the radio galaxy Hercules A (the central bright spot) and its two prominent radio lobes (the reddish-pink clouds), imaged by the Hubble Space Telescope and the Very Large Array. Image credit: nasa.gov.

The authors ingeniously sidestep this issue by looking at closeby pairs of light sources; by looking at the difference in Faraday rotation between two light sources embedded in the ICM, we probe only the intracluster fields separating the two sources – the intergalactic and Galactic contributions cancel out! Serendipitously, the Universe has provided us with an abundance of double light sources in the form of radio galaxies, whose bright pairs of lobes naturally arise as material ejected from these galaxies interacts with the surrounding ICM (see Figure 1). Figure 2 illustrates, schematically, the authors’ strategy to probe intracluster magnetic fields via the rotation measures of radio lobes.

Figure 2: Schematic diagram showing the observation of a radio galaxy embedded in the intracluster medium. Light emitted from the two radio lobes (labeled RM1 and RM2 to indicate their different rotation measures) passes through the magnetic fields of the ICM, the intergalactic medium, and the Milky Way before reaching the observer (far left). Image credit: Figure 1 in today’s paper.

Baffling B-fields from bygone bodies

Since the authors are interested in the evolution of intracluster fields across the lifetime of the Universe, they comb through archived radio telescope data from both the NRAO VLA Sky Survey (NVSS) and from recent literature to obtain rotation measures for double-lobed radio galaxies across a wide range of redshifts (in this context, redshift just tells us how far into the past we’re looking). When compiling their dataset of lobe pairs, the authors make careful cuts based on the distances between the lobes and the locations of the lobes relative to the Milky Way so as to minimize rotation measure contamination from intergalactic and Galactic fields – when we take the difference between the rotation measures of a given pair of lobes, we want this difference to reflect only the contribution from intracluster fields.

Ultimately, the authors select 387 pairs of lobes from NVSS and 197 pairs from the literature, with redshifts as high as 3 (meaning that the light we’re seeing from the farthest lobe is almost 11.5 billion years old). Plotting the pairwise rotation measure differences (and the statistical dispersion in these differences) yields Figure 3. To high confidence, the authors conclude that the RM differences in higher-redshift clusters are statistically higher than those in lower-redshift clusters, thus implying that intracluster fields were stronger in the past.

Figure 3: Plots of the pairwise rotation measure (RM) differences (top two rows) and the statistical dispersion in these differences (bottom two rows, showing two different ways of quantifying the dispersion) vs. redshift for the radio lobe dataset analyzed by the authors. Blue points represent pairs of lobes from the NVSS catalog, while red points represent pairs compiled from the literature. The right column shows a subset of the data with RM measurement uncertainties below a certain threshold. Image credit: Figure 6 in today’s paper.

The authors go a step further and use these rotation measures to estimate the typical intracluster field strength for clusters that existed more than seven billion years ago (roughly half the age of the Universe) – but, this only leads to more confusion: there was too little time between the beginning of the Universe and the formation of these clusters for strong magnetic fields to have grown via typical channels, like dynamos. Thus, the authors conclude that strong magnetic fields must have existed in the early Universe, prior to the formation of these clusters. While intracluster fields will provide useful constraints on the growth of magnetic fields in the early Universe, the ultimate origin of these fields continues to elude us.

And thus, the Universe’s grand magnetic mystery lives on.

Astrobite edited by: Catherine Manea

Featured image credit: nasa.gov

About Ryan Golant

I am a second-year astronomy Ph.D. student at Columbia University. My current research involves the use of particle-in-cell (PIC) simulations to study magnetic field growth in gamma-ray burst afterglows and closely related plasma systems. I completed my undergraduate at Princeton University, and am originally from Northern Virginia. Outside of astronomy, I enjoy learning about art history, playing violin and video games, and watching cat videos on the internet.

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