Astronomers have been puzzled for decades by mysterious structures in the sky. Now they might have an answer.
A vast network of magnetized filaments may connect the North Polar Spur to the Fan Region on opposite sides. They form a structure that looks like a tunnel and circles the Solar System.
Jennifer West, an astronomer at the University of Toronto, Canada, said that if she could see radio light, "we'd see this tunnel-like structure in almost every direction that we looked."
Although we have known the existence of the structures since the 1960s they are still difficult to comprehend. It's difficult to determine how far they are from us. Distances have varied from hundreds to thousands of light years away.
But, there had never been an analysis that linked the two structures together. West and her colleagues were able show that the two regions and the prominent radio loops between them could be linked. This solved many of the puzzles associated with each.
Comparative view of a tunnel with orientation. (Left: Pixabay/wal_172619/J. West; Right: Dominion Radio Astrophysical Observatory/Villa Elisa telescope/ESA/Planck Collaboration/Stellarium/J. West
"A few years back, Tom Landecker (one of our co-authors) told me about a 1965 paper that was published in the early days radio astronomy. The authors (Mathewson & Milne) speculated that the polarized radio signals could have arisen from our view of the Local Arm, which is located inside the galaxy.
"That paper inspired my to create this model and tie it to the much better data our telescopes provide us today."
The researchers used simulations and modeling to create a radio sky. They also played with distance parameters to find the best fit.
The team was able, using this information, to determine that the Solar System's most likely distance from the structures is approximately 350 light-years. This is consistent with other closer estimates. Based on Gaia data, the team also estimated the North Polar Spur's distance earlier this year. It was found that most of the spur lies within 500 light years.
West and her team measured the tunnel's length in approximately 1,000 light-years.
The North Polar Spur (top), and Fan Region (bottom) are light intensities. (West et al., arXiv, 2021)
This model agrees with a large range of observational properties in the North Polar Spur or Fan Region, such as the shape, the intensity, and the polarization (that is, how the electromagnetic radiation is twisted).
Bryan Gaensler, an astronomer at the University of Toronto, said that "this is extraordinarily clever work."
"When Jennifer first presented this idea to me, it seemed too outlandish to be an explanation. She was able to convince my! Now, I am excited to see what the rest of the Astronomy Community thinks.
Further work is required to confirm the findings and then model the structure more precisely. This may help solve a larger mystery, however: how galaxies form and evolve magnetic fields and how they are maintained. The researchers believe it could provide context for understanding magnetic filamentary structures around the galaxy.
Although the team plans to do more complicated modelling, they suggest that more sensitive and higher-resolution observations could reveal hidden details about how the structure fits in the larger galactic context.
"Magnetic fields can't exist alone. They must all be connected to one another. West stated that the next step is to understand how the local magnetic field connects to both the larger-scale galactic magnet field and the smaller-scale magnetic fields of the Sun and Earth.
"It's amazing to think that these structures are all around us, when we look up at the night sky.
The research will appear in The Astrophysical Journal. It is also available on arXiv.
Cover image credit: Dominion Radio Astrophysical Observatory/Villa Elisa telescope/ESA/Planck Collaboration/Stellarium/J. West
