double helix messier 87 galaxy
© Sophia Dagnello / NRAO / AUI / NSF / Pasetto et al.
VLA image of the Messier 87 radio jet, made at multiple radio frequencies. The jet seen in this image is about 8,000 light-years long. This image clearly shows the corkscrew-like helical structure in the inner part of the jet, which originates at the bright spot at the left, at the core of the galaxy, where a supermassive black hole resides.
Astronomers using NSF's Karl G. Jansky Very Large Array have captured unprecedented high-fidelity radio images of a jet of material propelled from the core of the giant elliptical galaxy Messier 87.

Messier 87 is an elliptical galaxy located some 53 million light-years from us in the constellation of Virgo.

Also known as M87, NGC 4486 or Virgo A, it hosts a black hole some 6.5 billion times more massive than the Sun at its center. Messier 87's black hole is the first one ever to be imaged — an achievement done with the Event Horizon Telescope (EHT) and announced in 2019.

Earlier this year, new EHT images traced the magnetic field in the vicinity of the black hole event horizon.

National Autonomous University of Mexico's Dr. Alice Pasetto and colleagues used the Very Large Array (VLA) to reveal details of the magnetic field by tracing the polarization, or alignment, of radio waves emitted from it, and by measuring the field's strength across different parts of the jet.

Their observations, made using the VLA's widest configuration that provides the highest resolution, produced very detailed images of the galaxy's jet.

"By making high-quality VLA images at several different radio wavelengths of the galaxy Messier 87 (M87), we were able to reveal the 3D structure of the magnetic field in this jet for the first time," Dr. Pasetto said.

"The material in this jet traces a double helix, similar to the structure of DNA."

"Helical magnetic fields are expected close to the black hole, and are thought to play a highly important role in channeling the material into a narrow jet, but we didn't expect to find such a strong helical field extending so far outward," added Dr. Jose Marti, an astronomer at the University of Valencia.
messier 87 galaxy double helix
© Pasetto et al
An analysis of the polarization properties in the conical jet of Messier 87 revealing a helical magnetic field configuration.
The magnetic field is expected to weaken with its distance from the black hole.

However, the astronomers suggested that instabilities in the flow of material within the jet could make the magnetic field more ordered at the distances seen in the new VLA images.

The instabilities produce regions of higher pressure which also compress the magnetic field lines. The authors think that this interaction between instabilities in the flow and the magnetic field is what produces the double-helix structure shown by the VLA images.

If this is happening in the Messier 87 jet, it likely also is happening in similar jets from galaxies throughout the Universe.

"Messier 87 is relatively near to us and its jet is very powerful, making it an excellent target for study," said Dr. Jose Gomez, an astronomer at IAA-CSIC.

"The clues it gives us can help us understand this very important and ubiquitous phenomenon in the Universe."

The findings were published in the Astrophysical Journal Letters.

Reference: Alice Pasetto et al. 2021. Reading M87's DNA: A Double Helix Revealing a Large-scale Helical Magnetic Field. ApJL 923, L5; doi: 10.3847/2041-8213/ac3a88