🔭 astrophysics

Magnetised CGM Gas at z~1 revealed by SPICE-RACS

By analyzing the largest sample to date of 2,483 quasar sightlines using SPICE-RACS and DESI data, this study reveals a significant excess in Faraday rotation measure dispersion around Mg II absorbers at $z \sim 1.14$ , providing direct evidence for substantial magnetic fields ( $\sim0.4 - 0.8\, \mu$ G) in the circumgalactic medium of galaxies by redshift 1.

Original authors: Sunil Malik, S. P. O'Sullivan, A. J. M. Thomson, C. S. Anderson, C. Van Eck, L. Rudnick, Amit Seta, B. M. Gaensler, Y. K. Ma, Takuya Akahori, D. Alonso-López, M. Brüggen, E. Carretti, S. W. Duchesne

Published 2026-05-19

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CC BY 4.0

Original authors: Sunil Malik, S. P. O'Sullivan, A. J. M. Thomson, C. S. Anderson, C. Van Eck, L. Rudnick, Amit Seta, B. M. Gaensler, Y. K. Ma, Takuya Akahori, D. Alonso-López, M. Brüggen, E. Carretti, S. W. Duchesne, T. J. Galvin, G. Heald, O. Hlinka, A. Khadir, S. A. Mao, R. Omae

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the space around a galaxy isn't empty. It's filled with a vast, invisible "halo" of gas, like a cosmic fog that stretches far beyond the galaxy's visible stars. Astronomers call this the Circumgalactic Medium (CGM). For a long time, we suspected this fog was threaded with magnetic fields, but proving it was like trying to find a specific needle in a haystack while wearing thick, blurry gloves.

Here is what this new study did to finally see those needles, explained simply:

The Problem: The "Static" on the Line

To measure magnetic fields in space, astronomers use a trick called Faraday Rotation. Imagine shining a flashlight (a radio signal from a distant quasar) through a foggy room. If the room has a magnetic field, the light twists slightly as it passes through. By measuring how much the light twists, we can calculate the strength of the magnetic field.

However, our view is blocked by our own galaxy, the Milky Way. It's like trying to listen to a faint radio station while standing next to a loud, crackling fire. The "static" from our own galaxy's gas and magnetic fields (called the Galactic RM) was drowning out the signal from the distant galaxies. Previous studies tried to subtract this static, but their "noise-canceling headphones" weren't good enough, leaving them with confusing or weak results.

The Solution: A Better Filter and a Bigger Net

The authors of this paper, led by Sunil Malik, decided to upgrade their equipment and their cleaning process:

A Bigger Net: They used a massive new survey called SPICE-RACS (using the ASKAP radio telescope in Australia) combined with a huge catalog of distant quasars from DESI. Instead of looking at a few dozen galaxies, they analyzed 2,483 lines of sight. This is like switching from looking at a single drop of rain to watching a whole storm.
The "Annulus" Trick: To remove the Milky Way's static, they used a clever method called an "annulus-based" subtraction. Imagine you are trying to hear a whisper in a noisy room. Instead of just guessing how loud the room is, you stand in a circle around the whisperer and measure the noise in the ring around them. You then subtract that ring's noise from the center. This gave them a much cleaner "residual" signal.
The "Clean Room" Cut: Even after subtracting the static, some "dust" remained in the form of dense gas clouds in our own galaxy. The team set strict rules: they threw away any data that passed through areas of our galaxy with high hydrogen gas or bright ionized gas (H-alpha). This left them with a pristine "clean sample" of 757 quasars.

The Discovery: The Fog is Magnetic

Once they had their clean data, they split the quasars into two groups:

Group A: Quasars whose light passed through a "Mg II absorber"—a cloud of gas in front of a distant galaxy (the target).
Group B: Quasars whose light passed through empty space with no galaxy in front of them (the control group).

The Result: The light from Group A twisted significantly more than the light from Group B.

The "twist" (magnetic signal) for the galaxies was 4.5 times stronger than what you'd expect from random chance.
This proves that the gas surrounding these distant galaxies (at a time when the universe was about half its current age, roughly 10 billion years ago) is indeed magnetized.

What Does This Mean?

Using some physics math (assuming the gas and magnetic fields are in a kind of balance), the team estimated the strength of these magnetic fields.

They found fields ranging from 0.4 to 0.8 microGauss.
To put that in perspective: Earth's magnetic field is about 50,000 microGauss. So, these fields are incredibly weak—like a whisper compared to a shout. However, in the vast, thin vacuum of space, this is a substantial amount of magnetism.

The Takeaway

This paper is a breakthrough because it finally provides solid proof that galaxies were already "magnetized" in their outer halos billions of years ago. It's not just the stars and the core of the galaxy that have magnetic fields; the entire cosmic neighborhood surrounding them does too.

The authors conclude that this magnetization likely happened early in the universe's history, possibly driven by winds blowing out from the galaxies themselves. This study doesn't just confirm the existence of these fields; it sets a new, cleaner standard for how we should measure them in the future, paving the way for even more detailed maps of the universe's magnetic web.

Technical Summary: Magnetised CGM Gas at z ∼1 revealed by SPICE-RACS

Problem Statement
Magnetic fields are theoretically expected to permeate the circumgalactic medium (CGM) of galaxies, influencing gas cooling, cosmic-ray confinement, and angular momentum transport. However, direct constraints on these fields at high redshift ( $z$ ) have remained limited due to a lack of high-quality Faraday rotation measure (RM) data. Previous studies utilizing RM measurements from the NRAO VLA Sky Survey (NVSS) or early Mg ii absorber samples suffered from large uncertainties, poorly constrained Galactic RM (GRM) modeling, and small sample sizes. These limitations led to disparate results, with some studies inferring $\gtrsim \mu$ G fields while others found $\lesssim \mu$ G strengths, necessitating a detailed investigation using larger samples of precise RMs.

Methodology
To address these limitations, the authors compiled the largest sample to date of quasar sightlines with associated RMs to probe magnetised CGM environments at $z \sim 1$ . The methodology involved the following steps:

Data Compilation: The study utilized Faraday RMs from the SPICE-RACS Data Release 2 (DR2), an ASKAP survey covering the southern sky and extending to $+49^\circ$ declination, cross-matched with the DESI DR1 spectroscopic quasar catalogue. This yielded a sample of 2,483 unique sources well-characterized at both optical and radio wavelengths.
Sample Classification: Sightlines were classified based on the presence of foreground Mg ii absorbers (tracing intervening galaxies) versus control sightlines without such absorbers. The initial sample contained 612 Mg ii sightlines and 1,871 control sightlines.
Foreground Mitigation: A critical component of the methodology was the rigorous removal of Milky Way contamination.
- GRM Subtraction: An annulus-based method was employed to estimate and subtract the Galactic RM contribution, defining the residual RM ($RRM = RM - GRM$).
- Density Cuts: To address residual contamination from high-density neutral and ionised interstellar medium phases, the authors applied strict cuts on Galactic HI column density ( $< 3.5 \times 10^{20} \text{ cm}^{-2}$ ) and H $\alpha$ intensity ( $< 1$ Rayleigh).
- Final Sample: These filters resulted in a "foreground-cleaned" sample of 757 quasars (191 with Mg ii absorbers and 566 controls) spanning redshifts $0.13 < z < 3.45$ .
Statistical Analysis: The authors compared the dispersion of $|RRM|$ between the Mg ii and control subsamples using the Median Absolute Deviation (MAD). They calculated the excess dispersion ( $\sigma_{excess}$ ) and performed a Bayesian model comparison to test the hypothesis that Mg ii sightlines exhibit a different RM dispersion than non-absorbers.
Physical Inference: To estimate magnetic field strengths, the authors combined energy equipartition assumptions between thermal gas and magnetic fields with a turbulent Faraday-screen model. They assumed a CGM temperature of $10^4$ K and an ionization fraction of 0.8.

Key Results

RM Excess: In the foreground-cleaned sample, sightlines with Mg ii absorbers exhibit a statistically significant excess in residual RM dispersion compared to the control sample. The excess is measured at $4.13 \pm 0.91 \text{ rad m}^{-2}$ , representing a $4.5\sigma$ detection at a median absorber redshift of $z \sim 1.14$ .
Bayesian Evidence: A Bayesian model comparison yielded a Bayes factor of 41.6, providing decisive evidence that the Mg ii subsample possesses a distinct RM dispersion relative to the non-Mg ii subsample.
Sensitivity to Cuts: The study demonstrates that without the specific HI and H $\alpha$ cuts, the RM excess is statistically insignificant ( $<1\sigma$ ), highlighting the necessity of removing high-density Galactic regions to isolate the extragalactic signal.
Magnetic Field Strength: Assuming energy equipartition and a turbulent screen model with a path length ( $L$ ) of 20–150 kpc, the observed excess implies CGM magnetic field strengths of approximately $0.4 - 0.8 \mu$ G. This corresponds to equipartition electron densities of $\approx 10^{-2}$ to $10^{-3} \text{ cm}^{-3}$ .

Significance and Claims
The paper claims to provide the first robust, statistically significant detection of magnetised gas in the CGM of high-redshift galaxies using a large, high-precision RM sample. The authors assert that their results indicate substantial CGM magnetisation was already established by $z \sim 1$ .

The study emphasizes that previous discrepancies in the field were largely due to insufficient foreground modeling and small sample sizes. By employing a conservative and homogeneous methodology—including improved GRM subtraction and strict Galactic foreground cuts—the authors successfully isolated the extragalactic signal. They conclude that normal galaxies host a magnetised CGM that appears larger than predicted by some current simulations.

The authors note several caveats, including the dependence of field strength estimates on poorly constrained CGM properties (density, path length, coherence) and the fact that Mg ii absorbers may preferentially trace denser, metal-enriched gas rather than the full CGM volume. Consequently, the derived field strengths are model-dependent. However, the work establishes a framework for future studies using forthcoming high-density RM grids from POSSUM and the SKA to resolve the coherence scale, geometry, and redshift evolution of magnetic fields in galaxy halos.

The Problem: The "Static" on the Line

The Solution: A Better Filter and a Bigger Net

The Discovery: The Fog is Magnetic

What Does This Mean?

The Takeaway

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