What we thought was a pretty normal spiral galaxy not far from the Milky Way has revealed a hidden surprise.
NGC 4632, some 56 million light-years away, is circled by a huge ring of gas that wraps around the galaxy at a highly inclined angle to its galactic plane. Why didn't we see it until now? It's invisible in most of the electromagnetic spectrum, appearing only when we stare at the sky with radio telescopes.
The discovery could put NGC 4632 in a class of extremely rare galaxies known as polar ring galaxies – but also suggests that these galaxies might not be as rare as we thought. Rather, NGC 4632 could mean that we've just been looking at them in the wrong light.
"The findings suggest that one to three per cent of nearby galaxies may have gaseous polar rings, which is much higher than suggested by optical telescopes. Polar ring galaxies might be more common than previously thought," says astrophysicist Nathan Deg of Queens University in Canada.
"While this is not the first time astronomers have observed polar ring galaxies, NGC 4632 is the first observed with ASKAP and there may be many more to come."
Polar ring galaxies are pretty much what they sound like: galaxies with a ring of material – dust, gas, and stars – that orbits around, or close to, the galaxy's poles; that is, perpendicular to the galactic plane.
They usually look pretty spectacular, with their glittering rings skewed like the torus of a von Braun space station.
We don't know how they got that way, but there are two current leading contenders.
The first is that material travels along the cosmic web through intergalactic space and ends up in orbit around a galaxy, forming part of it.
The second, more commonly accepted explanation is that the ring is made up of material gravitationally slurped from another galaxy that came close to and interacted with the polar ring galaxy.
Interestingly, it tends to be galaxies of the lenticular and elliptical types that sport polar rings. These are galaxies that are fuzzy and unstructured, without the well-defined spiral arms seen in galaxies like the Milky Way.
Optical observations have revealed polar rings around roughly 0.5 percent of the nearby lenticular galaxies. But there's so much more to the Universe than meets our limited human eye.
Even so, Deg and an international team of astronomers weren't necessarily expecting a hidden polar ring around spiral galaxy NGC 4632.
It appeared in data collected by the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) conducted using the Australian Square Kilometer Array Pathfinder, a powerful radio telescope array located in the desert of Western Australia. The goal of WALLABY is to survey hundreds of thousands of galaxies in the southern sky, in order to map their gas distribution.
"NGC 4632 is one of two polar ring galaxies we've identified from 600 galaxies that were mapped in our first small WALLABY survey," says astronomer Bärbel Koribalski of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia.
"Using ASKAP over coming years we expect to reveal more than 200,000 hydrogen-rich galaxies, among them many more unusual galaxies like these ones with polar rings."
NGC 4632 doesn't tell us immediately how polar ring galaxies get their rings. But it – along with the second polar ring spiral galaxy the team found, NGC 6156 – suggests that there may be a large population of them out there that we're yet to find.
It could also suggest the formation mechanism for polar ring spiral galaxies is different from that of previously identified polar ring galaxies.
The team hopes that further observations will reveal more of these mysterious objects. This should reveal whether the type of ring – gaseous, or star-filled – is linked to the structure of the host galaxy. As will taking closer looks at NGC 4632 and NGC 6156.
"As better observations and more sophisticated models are obtained for both galaxies, it will be possible to constrain the parameters of the ring progenitor (if they are indeed formed via mergers or flybys)," the researchers write.
"Understanding whether these are from interactions or gas accretion will provide constraints on galaxy formation and evolution."
The research has been published in the Monthly Notices of the Royal Astronomical Society.