‘Spectacular’ Polar Ring Galaxies May Be More Common Than Thought, Study Suggests

Astronomers have found two candidates for this rare type of galaxy, surrounded by a halo of hydrogen gas—and they could provide insights about dark matter

A blue main disk of a galaxy surrounded by a white ring of hydrogen gas.
A potential polar ring galaxy called NGC 4632. The composite image combines a capture of the galaxy's main disk, taken with the Subaru Telescope, with radio wave data of the hydrogen ring, which has been digitally colorized as white. Jayanne English (U. Manitoba), Nathan Deg (Queen's University) & WALLABY Survey, CSIRO / ASKAP, NAOJ / Subaru Telescope

Astronomers have found two cosmic structures that may be examples of rare polar ring galaxies.

Researchers used an array of telescopes in Western Australia to peer beyond the Milky Way and study hydrogen gas in galaxies across the cosmos. The two potential polar ring galaxies—in which a ring of hydrogen gas orbits the main spiral disk at a 90-degree angle—are highly unusual objects.

“These are some of the most spectacular-looking galaxies that we see,” Nathan Deg, an astrophysicist at Queen’s University in Canada and one of the researchers involved in the findings, tells Jack Ryan of the Australian Broadcasting Corporation (ABC).

The two galaxies have not yet been confirmed as polar rings, the researchers note in their new paper, published Wednesday in the Monthly Notices of the Royal Astronomical Society. But if verified, the findings would suggest polar ring galaxies are much more common than astronomers once thought.

The research comes from an effort called the WALLABY survey, which aims to detect and image the distribution of a neutral type of hydrogen to better understand the formation and evolution of other galaxies. The survey utilizes a radio telescope, called ASKAP, made up of 36 dishes, each measuring 12 meters wide, for detecting radio waves emitted by the hydrogen gas.

Together, the telescopes act as a single dish with a six-kilometer diameter, Bärbel Koribalski, a co-author of the study and a senior research scientist at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia, tells the Sydney Morning Herald’s Angus Dalton. ASKAP is located on the land of the Wajarri Yamaji, an Indigenous group, in Western Australia.

Using this telescope, the researchers discovered ribbons of hydrogen gas circling each of the galaxies, called NGC 4632 and NGC 6156. The galaxy shown in the above image, NGC 4632, could have acquired its hydrogen ring by sucking up the gas from a nearby, smaller galaxy, Koribalski writes in the Conversation.

“The bigger galaxy is kind of bullying the neighbor, it’s taking away the gas,” Koribalski tells the Sydney Morning Herald. “And depending on where they are in the sky and how they rotate, the gas might be created into a larger disc, or in this case, it formed a polar ring around the galaxy.”

Polar ring galaxies could also result from massive collisions between two galaxies in the distant past, according to NASA.

Astronomers reveal cosmic ribbon around rare galaxy

To depict NGC 4632, the researchers created a composite image using the radio wave data from ASKAP, which captured the ethereal ring, and an optical image of the galaxy from the Subaru Telescope in Hawaii, per NASA. Using a specialized computer program, the team removed hydrogen from the central reigon of the galaxy in the image, leaving a two-dimensional ring they colored white in a way that depicts its orbital motion.

The paper examines two of the 600 galaxies observed in the WALLABY pilot study, and in the future, the researchers anticipate they’ll be able to look at more than 200,000 hydrogen-rich galaxies.

Based on previous research, astronomers had suggested that polar rings surround just 0.1 percent of all galaxies, per the ABC. But now, the new study posits these unusual structures may be a little more common, comprising 1 to 3 percent of nearby galaxies, Deg says in a statement.

The findings could also yield information about dark matter. Dark matter, which makes up an estimated 27 percent of the universe, has never been observed, but it’s scientists’ explanation for what holds galaxies and clusters of galaxies together. Astronomers suspect an invisible ring of dark matter encircles most galaxies, Koribalski writes in the Conversation. So, by studying the motion and placement of the hydrogen gas halos in polar ring galaxies, researchers could potentially explore how this affects the structure of surrounding dark matter.

Kenneth Freeman, an astronomer at the Australian National University who did not contribute to the findings, tells the ABC that he’s not convinced the researchers have found polar ring galaxies. But he is intrigued by the scientific potential of the discovery.

“The real interest, certainly for me, is just what it can tell you about dark matter,” he says to the publication.

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