Hidden Polar Ring Discovered in Nearby Spiral Galaxy
Scientists have made an astonishing discovery in a seemingly ordinary spiral galaxy located 56 million light-years away from the Milky Way. The galaxy, known as NGC 4632, was found to be surrounded by a massive ring of gas that encircles the galaxy at a highly inclined angle to its galactic plane. Surprisingly, the ring is invisible in most of the electromagnetic spectrum and can only be seen using radio telescopes.
This remarkable finding suggests that NGC 4632 could belong to a rare class of galaxies called polar ring galaxies. However, it also challenges the belief that such galaxies are extremely uncommon. Astrophysicist Nathan Deg from Queens University in Canada explains, “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.”
Polar ring galaxies are characterized by a ring of material, including dust, gas, and stars, that orbits around or near the galaxy’s poles, perpendicular to the galactic plane. These galaxies typically exhibit a stunning appearance, with skewed rings resembling the torus of a von Braun space station.
There are two leading theories attempting to explain the formation of polar rings. The first one proposes that material moves through intergalactic space along the cosmic web and eventually becomes part of the galaxy, forming the ring. The second theory, which is more widely accepted, suggests that the ring consists of material gravitationally pulled from another galaxy that closely interacted with the polar ring galaxy.
Interestingly, polar rings are predominantly observed around lenticular and elliptical galaxies, which lack the well-defined spiral arms observed in galaxies like the Milky Way. Optical observations have so far revealed polar rings around approximately 0.5 percent of nearby lenticular galaxies. However, the recent discovery of a hidden polar ring surrounding NGC 4632 raises the possibility of many more yet to be found.
The discovery was made during the Widefield ASKAP L-band Legacy All-sky Blind survey (WALLABY) using the Australian Square Kilometer Array Pathfinder. WALLABY aims to map the gas distribution in hundreds of thousands of galaxies in the southern sky. Astronomer Bärbel Koribalski of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia explains, “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.”
Although the discovery of NGC 4632 does not immediately shed light on the formation mechanism of polar ring galaxies, it suggests the existence of a substantial population of these galaxies that are yet to be discovered. NGC 4632, along with another polar ring spiral galaxy called NGC 6156, could also indicate that the formation mechanism for polar ring spiral galaxies differs from previously identified polar ring galaxies.
The research team hopes that further observations will uncover more of these enigmatic objects. By studying the structure of NGC 4632 and NGC 6156 in detail, scientists aim to determine if the type of ring, whether gaseous or star-filled, is related to the host galaxy’s overall structure. This could provide insights into the formation and evolution of galaxies.
The findings have been published in the Monthly Notices of the Royal Astronomical Society.