A colourised radio telescope image of the central disk of NGC 4632 surrounded by a thick band of hydrogen gas (white). Photo: Supplied: Jayanne English (U. Manitoba), Nathan Deg (Queen's University) & WALLABY Survey. CSIRO/ASKAP radio telescope, NAOJ/Subaru Telescope
Searching through a catalogue of more than 600 galaxies, Nathan Deg stumbled upon two that seemed perfectly gift-wrapped.
Dr Deg, an astrophysicist from Queen's University in Ontario, Canada, was analysing data from the Australian Square Kilometre Array Pathfinder (ASKAP) telescope array in Western Australia, when he was struck by the characteristics of galaxies NGC 4632 and NGC 6156.
A ribbon of thick hydrogen gas surrounding the galaxies was the first hint he may have found an exceedingly rare and beautiful type of cosmic structure, known as a polar ring galaxy.
"These are some of the most spectacular looking galaxies that we see," Deg said.
The potential discovery of two polar ring galaxies, led by Deg, was published in the journal Monthly Notices of the Royal Astronomical Society.
Polar ring galaxies typically appear as a central disk of stars, gas and dust enclosed by a second, perpendicular ring - like a frisbee zipping through a hula hoop.
One of the most famous examples, NGC 4650a, was imaged by the Hubble telescope in 1999.
But they are tough to find. Estimates from previous research suggest they make up about only 0.1 percent of galaxies.
Finding two in a sample of 600 galaxies was a total surprise, Deg said.
Though the evidence suggests these may be polar rings, the researchers are quick to point out the stunning structures could have other explanations.
"Some spiral galaxies with large warped disks can look like polar rings," said Bärbel Koribalski, an astronomer at CSIRO and co-author on the new study.
If they are confirmed as polar ring galaxies, they present a way to understand how these curious and rare galaxies form and evolve.
They may even help provide a way to understand dark matter, an elusive force that permeates the cosmos.
Put a ring on it
The two potential polar ring galaxies were spotted in the first data release from WALLABY, an all-sky survey that uses ASKAP's 36 antennas to look for a specific type of hydrogen across the universe.
The hydrogen signal provides a way to map the structure of gas inside and outside of galaxies and can offer clues about how that gas moves.
"With ASKAP, we see both where the gas is on the sky, but also how fast the gas is moving towards or away from us," Deg said.
The WALLABY survey presented an opportunity to study the dynamics of the hydrogen gas surrounding NGC 4632 and NGC 6156 in more detail.
NGC 4632 was first spotted by the survey in 2022 and originally described as a spiral galaxy situated in the Virgo constellation, lying about 56 million light-years away. Of the two galaxies, its hydrogen ring is the most obvious.
Because the hydrogen gas is detectable only at radio wavelengths that can't be seen by the human eye, images of NGC 4632, like the one at the top of this article, have been digitally colourised in white. The hydrogen gas from the inner galaxy has also been removed, to bring out the ring shape.
Further computer modelling helped the research team simulate the dynamics of NGC 4632, exploring how the gas moves around. A polar ring seemed to be the best explanation for the mass of gas surrounding it.
NGC 6156 was a different story. It lies about 150 million light-years away and its ring is much less obvious.
In this case, the researchers had to look at the speed of the gas moving around the galaxy. Again, their models seemed to suggest this could be a polar ring, though with less certainty.
"We do need to confirm these fully as being polar," Deg noted.
If the discovery of the polar rings is confirmed, it also suggests these types of galaxies are much more common than previously thought.
The find highlights the promise of ASKAP and its capabilities.
"The two polar ring galaxies are just the beginning of a huge project," Professor Koribalski said.
In total, WALLABY hopes to spot upwards of 200,000 galaxies.
With a bounty of curiosities to delve into, ASKAP then provides an opportunity to help answer some of the Universe's most burning questions.
Questions like ... where's all the dark matter?
Into the dark
Astronomers have postulated that polar ring galaxies might be created when a larger galaxy eats a smaller one in just the right way.
Or, perhaps, they're formed when two galaxies stray a little too close to one another.
But no matter how they're formed, polar ring galaxies provide a unique opportunity to study dark matter, the invisible force that seems to bind the cosmos together.
Kenneth Freeman, a professor of astronomy at Australian National University, who was not associated with the research, is not convinced the galaxies are definitely polar ring galaxies.
"But the real interest, certainly for me, is just what it can tell you about dark matter," Freeman said.
That's because typical disk and spiral galaxies are assumed to have a spherical halo of dark matter surrounding them.
With a polar ring galaxy, the shape of the dark matter halo will be affected by the two perpendicular disks, giving researchers a way to probe the nature of that halo.
"That's almost impossible with any other technique," Freeman said.
"We've never really successfully done it, but that's the goal."
- This story was first published by the ABC.