Corkscrew Galaxy: How Twisted Radio Jets Probe Magnetic Fields in Clusters
We present polarimetric observations of a radio galaxy with a spiral jet (the "Corkscrew Galaxy") at 1296–1440 MHz with the ASKAP telescope (POSSUM survey). We tested the hypothesis linking quasi-periodic oscillations of the jet on the sky plane to Faraday rotation. Significant rotation measure (RM) oscillations were detected with a spatial period of (342±101) arcseconds, which, within the Rayleigh resolution, matches the lateral deviations of the jet of (290±72) arcseconds. Cross-correlation analysis revealed a systematic variation: in the eastern section, jet deviations and RM are in phase, indicating a jet-associated or sheath Faraday screen, while in the west a phase shift points to a change in the dominant rotating medium, with a possible contribution from the intracluster medium. We show that quasi-periodic RM signatures can separate magnetic field sources in AGN jets, and that for some embedded radio galaxies the main RM contribution comes from the local medium near the source rather than the cluster foreground. We outline reliability criteria to rule out spurious detections.
Context
Magnetic fields play a key role in launching, collimating, and interacting jets from supermassive black holes at the centers of galaxies, whose redshift indicates the expansion of the Universe, discovered by Edwin Hubble, with the intergalactic medium. However, determining exactly where Faraday rotation occurs—in the jet itself, its sheath, or the background cluster plasma—remains a challenge. Spiral or corkscrew radio galaxies, where the jet undergoes regular lateral deviations, offer a unique opportunity: if Faraday rotation is linked to the jet morphology, then by analyzing the periodicity of RM and its phase relationship with the bends, we can 'untangle' the contribution of different magnetized media. This is especially important for understanding heating mechanisms and the growth of entropy in clusters, where one must also consider the mass determined by gravitational lensing, and the distribution of dark matter, first noted by Fritz Zwicky.
Methods
Observations were made with the Australian ASKAP radio telescope as part of the POSSUM survey in the 1296–1440 MHz band. For the bright radio galaxy ESO 137-G007 ('Corkscrew Galaxy') in the Norma Cluster (Abell 3627), a high signal-to-noise Faraday rotation measure (RM) map was constructed. The method is based on the dependence of the polarization angle on wavelength, described by the constant speed of light, which enters the formula derived from James Clerk Maxwell's equations. Using the FilFinder algorithm, they extracted the jet's ridge line and measured its transverse deviations. Then they constructed one-dimensional RM profiles along the axis and, applying the Lomb-Scargle periodogram and cross-correlation analysis, compared them with the deviation series. To exclude false signals, they created mock RM maps reproducing galactic foreground fluctuations and used phase-randomized surrogate data.
Results
It turned out that the RM along the jet varies regularly, with an amplitude of up to several hundred rad/m² and a characteristic spatial period of (342±101) arcseconds, which, within the Rayleigh resolution, matches the period of lateral jet deviations of (290±72) arcseconds. The RM peak in the periodogram is 11 times higher than the 84th percentile of the null model distribution, excluding a galactic foreground origin. Cross-correlation revealed different behavior in the eastern and western sections: in the eastern part, the RM and jet deviations are almost in phase (only 7.6% of null models yield a higher correlation at zero lag), while in the western part, the connection is weak, with a shifted phase. This suggests a transition from a jet or sheath Faraday screen to the dominance of the local intergalactic medium, rich in hydrogen and helium. Magnetic field estimates yield a few microgauss for both the jet cocoon and the surrounding plasma.
Implications
The results clearly demonstrate for the first time that the spiral structure of a radio jet can serve as a 'ruler' to separate internal and external sources of Faraday rotation. This opens up the prospect of using a population of corkscrew radio galaxies as sensitive probes of the magnetized intracluster medium, allowing magnetic field parameters to be measured on scales of tens of kiloparsecs. Moreover, the approach provides a new observational test for models of jet formation and stability, including the role of Kelvin-Helmholtz and kink instabilities, which also affect the distribution of entropy.
Future development
With the commissioning of the Square Kilometre Array (SKA), the discovery of a large number of galaxies with pronounced spiral jet structures is expected. For these, statistical analysis will become possible: for example, comparing RM at points of maximum bending and at midpoint for a large sample will reveal how often jet or environmental magnetic fields dominate. Detailed magnetohydrodynamic (MHD) modeling including polarized radiative transfer, including synchrotron emission and Faraday rotation calculations, will be the next step to refine the interpretation of the observed phase relationships. In parallel, comparison with gravitational lensing maps and dark matter distributions will help link magnetic fields to the total mass of clusters.
Impact
The work will impact the physics of active galactic nuclei, cluster astrophysics, and the methodology of next-generation polarimetric surveys. The ability to distinguish the contributions of jet and environmental magnetic fields is critical for understanding AGN feedback and the evolution of the large-scale structure of the Universe.
Next steps
Next steps include multi-band polarimetric observations of the Corkscrew Galaxy with MeerKAT and the future SKA to test the model with broader frequency coverage and better resolution. An expansion of the sample of spiral sources for statistical analysis of RM–morphology is also planned.
Key open problems
The study is directly connected to two key unsolved problems: the origin and stability of relativistic jets ( how spiral structures form and are maintained) and the nature of cosmic magnetism (how magnetic fields are amplified and ordered in galaxy clusters). Additionally, it touches on the fundamental question of how dark matter interacts with ordinary matter through magnetic fields.
🎯 It got its name 'Corkscrew Galaxy' because of its striking resemblance to a corkscrew: its radio jet, nearly 600 kiloparsecs long, is twisted into two full turns! This is one of the longest known spiral jets in the Universe.
Key numbers
- RM oscillation period: (342±101) arcseconds
- jet bending period: (290±72) arcseconds
- radio jet extent: ~570 kiloparsecs
- magnetic field in the western part: 2–4 μG
- electron density in the ICM: (6–9)×10⁻⁴ cm⁻³