New observations have revealed plasma strung along vast magnetic “strings” crossing hundreds of thousands of light-years.

At the center of the Norma galaxy cluster, 220 million light-years away in the Southern Hemisphere sky, is a large, bright galaxy. A supermassive black hole at its core powers two beams of particles and radiation that shoot out of the galaxy and plow straight into the surrounding gas. Even as they bloom and billow as if in hurricane winds, the jets press on for hundreds of thousands of light-years.

As if that weren’t fantastical enough, now a radio array in South Africa known as MeerKAT has revealed three “strings” that somehow connect the two jets. The strings, strung from one lobe to the other like hand-pulled Chinese noodles, stretch as long as 260,000 light-years across extragalactic space.

Plasma strung across magnetic strings connect two jet lobes across hundreds of thousands of light-years. The jet lobes dwarf the galaxy, which is a pinpoint at the center. Mpati Ramatsoku and colleagues identified three strings: a bright one that extends straight across (the study does not identify the wisp above it as a string), a second straight one that appears disconnected, and a third one that curves like a backward "c" on the left.
Rhodes University / INAF / SARAO

While not entirely unprecedented, strings like this have never been seen at this level of detail, and the results invite speculation as to the strings’ origin. Mpati Ramatsoku (Rhodes University, South Africa, and INAF–Astronomical Observatory of Cagliari, Italy) and colleagues have published the observations in the April issue of Astronomy & Astrophysics.

Are the Strings Real?

MeerKAT is an array of 64 radio antennas, each the size of a semitrailer, that worked together to image the galaxy ESO 137-006 at radio frequencies of 1 GHz and 1.4 GHz.  As a precursor to the upcoming Square Kilometer Array being built in South Africa and Australia, MeerKAT provides unprecedented resolution at these frequencies.

Previous instruments had hinted at string-like structures between jets in a couple previous sources, perhaps most notably a single filament connecting the jets extending to either side of the radio galaxy 3C 338. But that filament could be the remnant of an older jet. MeerKAT’s sharp image of ESO 137-006 provides unambiguous evidence of strings that are separate from the jets they connect.

Gregory Taylor (University of New Mexico), who was not involved in the study, says the results can be taken at face value: “I don’t see any way for typical errors that we encounter in interferometry (such as radio frequency interference or calibration errors) to generate these features.” Supporting the features’ credibility, he adds, are the number and variety of filaments within both lobes, both straight and curved.

Plasma on a Magnetic String

If the features are real, the question remains: What are they?

Ramatsoku and colleagues’ analysis of the radio data determined that the radio waves they give off come from electrons spiraling as they whiz along magnetic field lines. This is a common type of emission from jets called synchrotron radiation.

Furthermore, the data suggest all three strings formed around the same time. Electrons tire of their spiraling, slowing down as they radiate energy, which changes the nature of the emission over time. But the strings revealed in the MeerKAT data all have the same kind of synchrotron emission. So, in a nutshell, the strings amount to magnetic lines strung with plasma, all three of them born at the same time.

Radio and X-ray image of magnetically connected galaxy
The same radio data shown above (orange-red) is superimposed on an X-ray image (blue), which shows the thin gas permeating the center of the Norma Cluster.
M. Murgia (INAF) / SARAO

These results still leave a lot of room for speculation, though, and the researchers suggest some possible scenarios. For example, as the jet-shooting galaxy pushes through the thin gas around it, it could be that the blooming jet lobes interact somehow with this gas, trailing magnetic field lines and plasma behind them. But the speculative scenarios need further study, the researchers say, both in terms of theoretical work and additional observations.

“I think the filaments in ESO 137-006 are telling us about exceptionally strong magnetic fields at play in these radio lobes,” Taylor says. “Still, it is amazing that these features can be maintained [over vast distances and times] without being broken up by the passage of other galaxies in the cluster.”


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