Giant hydrogen band provides evidence of rare polar ring galaxy

A team of astrophysicists stumbled upon two potential galaxies while scourging through a catalogue consisting dataset of over 600 galaxies. 
Mrigakshi Dixit
The featured composite image combines this gas ring, observed with the highly sensitive ASKAP telescope, with optical data from the Subaru telescope.
The featured composite image combines this gas ring, observed with the highly sensitive ASKAP telescope, with optical data from the Subaru telescope.

Jayanne English (U. Manitoba), Nathan Deg (Queen's University) & WALLABY Survey, CSIRO/ASKAP, NAOJ/Subaru Telescope 

Astronomers have discovered possible evidence of a rare galaxy type known as "polar ring galaxies."

Polar ring galaxies are an intriguing class with a distinct and perplexing structure. These galaxies are distinguished by a ring of stars, gas, and dust significantly tilted to the galaxy's main disk. 

A team of astrophysicists stumbled upon two potential galaxies while scourging through a catalogue consisting of a dataset of over 600 galaxies. 

As per the Conversation, the comprehensive dataset was gathered by the high-sensitive Australian Square Kilometre Array Pathfinder (ASKAP) telescope array in Western Australia. 

Finding polar ring galaxies is rare

Among the galaxies in this collection, NGC 4632 and NGC 6156 appeared to have different structural features than the others. 

Upon first observation, they noticed an enormous band of dense hydrogen gas around the NGC 4632 galaxy. This evidence hinted at the existence of a polar ring galaxy.

Polar ring galaxies can be challenging to detect and study because their polar rings are often faint and do not release as much light as the core disk. Reportedly, estimates suggest that polar ring galaxies account for just approximately 0.1 percent of galaxies. 

Observations at several wavelengths, such as radio and infrared, are essential for comprehensively understanding this class of galaxies. 

Finding two in a sampling of 600 galaxies was thus pure chance. Scientists focused on NGC 4632 because its structure looked to be more apparent.

“Galaxy NGC 4632 hides a secret from optical telescopes. It is surrounded by a ring of cool hydrogen gas orbiting at 90 degrees to its spiral disk. Such polar ring galaxies have previously been discovered using starlight. However, NGC 4632 is among the first in which a radio telescope survey revealed a polar ring,” mentioned NASA blog post. 

3D visualization of the galaxy's structure

The first data of 600 galaxies was released from the pilot WALLABY survey, which scans the entire sky using ASKAP's 36 antennas to detect hydrogen signals from celestial objects across the universe specifically.

NGC 4632 was discovered by the survey in 2022, but it was initially characterized as a spiral galaxy in the direction of the Virgo constellation.

A closer examination further revealed the presence of a distinct ring-like structure around the galaxy's main body. Following observation, astronomers conducted a detailed analysis using multiple advanced methodologies.

Among them, one was computational modeling to simulate the dynamics of this peculiar ring-like structure and better understand how the gas moves around the central galactic disk. Furthermore, scientists used 3D visualization tools such as iDaVIE to analyze the shape of the NGC 4632 galaxy.  

This demonstrated that a polar ring was the most likely explanation for the amount of hydrogen gas rotating over the poles of the NGC 4632 galaxy’s disk.

“The gas in the ring, which makes up about half of the system’s mass, was likely hoovered up from a companion galaxy,” mentioned the Conversation.  

The formation of polar ring galaxies is still a subject of study and debate among astronomers; however, one hypothesized process stems from the interaction and merger of two galaxies

This is only the beginning of the Wallaby survey, which has the potential to uncover over 200,000 hydrogen-rich galaxies. The finding of more similar galaxies will eventually broaden our knowledge of the peculiar nature of polar ring galaxies.

They can also provide information on galaxies' overall structure and evolution throughout the universe. Furthermore, studying these galaxies may bring scientists closer to understanding the invisible dark matter that may surround most galaxies.

The study results were reported in the journal Monthly Notices of the Royal Astronomical Society.

Study abstract:

We report on the discovery of two potential polar ring galaxies (PRGs) in the WALLABY Pilot Data Release 1 (PDR1). These untargeted detections, cross-matched to NGC 4632 and NGC 6156, are some of the first galaxies where the H Iobservations show two distinct components. We used the IDAVIE virtual reality software to separate the anomalous gas from the galactic gas and found that the anomalous gas comprises ∼50 per cent of the total H I content of both systems. We have generated plausible 3D kinematic models for each galaxy, assuming that the rings are circular and inclined at 90° to the galaxy bodies. These models show that the data are consistent with PRGs but do not definitively prove that the galaxies are PRGs. By projecting these models at different combinations of main disc inclinations, ring orientations, and angular resolutions in mock data cubes, we have further investigated the detectability of similar PRGs in WALLABY. Assuming that these galaxies are indeed PRGs, the detectability fraction, combined with the size distribution of WALLABY PDR1 galaxies, implies an incidence rate of ∼1–3 per cent. If this rate holds true, the WALLABY survey will detect hundreds of new polar ring galaxies.

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