What happens when one galaxy swallows another galaxy?
September 13, 2023
When looking at the sky using data from optical and radio telescopes, astronomers may not know exactly what they are looking for, but they’re well trained to identify abnormalities that might indicate a new discovery. That’s what happened recently when Queen’s researchers Nathan Deg and Kristine Spekkens (Physics, Engineering Physics, and Astronomy) were analyzing data obtained from a telescope owned and operated by CSIRO, Australia’s national science agency.
Looking at sky maps of over 600 galaxies, they identified two potential polar ring galaxies, a type of galaxy that exhibits a ring of stars and gas perpendicular to its main spiral disk. Although this is not the first time that astronomers have observed polar ring galaxies, they are the first observed with CSIRO’s ASKAP radio telescope, and these new detections in gas alone suggest polar ring galaxies might be more common than previously believed.
"Polar ring galaxies are some of the most spectacular looking galaxies in the Universe," says Dr. Deg, who is the lead author of the paper published in the Monthly Notices of the Royal Astronomical Society. "These 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."
The findings build on data from the WALLABY survey, an international project to detect atomic hydrogen emission from about half a million galaxies, covering most of the Southern sky.
Further investigation of polar ring structures can lead to a better understanding of how galaxies evolve. For example, one of the main hypotheses to explain the origin of polar rings is a merger where a larger galaxy 'swallows' a smaller one. If polar ring galaxies are more common than previously thought, this could mean that these mergers are more frequent.
In the future, polar rings can also be used to deepen our understanding of the Universe, with potential applications in dark matter research. "It's possible to use polar rings to probe the shape of the distribution of dark matter associated with the host galaxy, which could lead to new clues about the mysterious properties of dark matter," says Dr. Deg.
Turning data into impressive images
The new potential polar ring galaxy image was generated by Jayanne English (Associate Professor of Astronomy at the University of Manitoba and former postdoctoral fellow at Queen’s) using a combination of data from different telescopes. First, optical and infrared data from the Subaru telescope in Hawai’i provided the image for the spiral disk of the galaxy. Then, the gaseous ring around it was added based on data obtained from WALLABY.
The creation of this and other astronomical images are all composite because they include information that our eyes can’t capture. In this particular case, the cold hydrogen gas component, invisible to the human eye, is seen in radio "light" using ASKAP. The subtle colour gradient of this ring represents the orbital motions of the gas, with purple-ish tints tracing gas that moves towards the viewer. The light from the ring was separated from that emanating from the disk of the galaxy using virtual reality tools, in collaboration with Professor Tom Jarrett (University of Cape Town, South Africa).
"We were able to work with data that showed a fine grid of velocity channels, which are equivalent to the radio stations on your old-fashioned radio receiver. The richness of the velocity data meant we could assign multiple colours to this composite to subtly convey the motion happening within the polar ring," explains Dr. English. "The dance and choreography of the gas are beautiful, and that motion of the gas gives us some clues as to how galaxies evolve over time."
International collaboration to map the sky
Drs. Deg, Spekkens, English, and Jarrett are members of the WALLABY survey collaboration, with Dr. Spekkens sitting on the executive team. They worked with 27 collaborators from Canada, Australia, South Africa, Ecuador, Burkina Faso, Germany, China, and beyond to analyze the data from WALLABY’S first public data release, resulting in the newly published paper.
ASKAP is located at Inyarrimanha Ilgari Bundara, CSIRO’s Murchison radio astronomy observatory on Wajarri Yamaji Country in Western Australia, and its high sensitivity and resolution make the 36-dish telescope a perfect tool to discover new galaxies. As new data sets are analyzed (there’s an expectation to scan hundreds of thousands of galaxies), scientists are hoping to find more and more examples of polar rings and then confirm the findings using data from different telescopes, including the MeerKAT radio telescope in South Africa.
"Our ASKAP radio telescope is delivering a flood of data and we are ready for it," says Bärbel Koribalski, science leader at CSIRO. "Using ASKAP, full WALLABY survey will deliver more than 200,000 hydrogen-rich galaxies, among them many unusual objects like polar rings."
This discovery, along with many others being made by the ASKAP radio telescope, are paving the way for the SKA telescopes, a next-generation radio astronomy-driven Big Data facility being built in Australia and South Africa by SKA Observatory (SKAO).
Earlier this year, the Canadian federal government announced its intention to become a full member of the SKAO. Dr. Spekkens, Canada’s SKA Science Director and vice-chair of the SKAO Science and Engineering Advisory Committee, highlights that this move will provide generations of Canadian researchers access to the world’s biggest radio telescope.
"The polar rings that we found are just a taste of what WALLABY will deliver with the ASKAP radio telescope, which is just a taste of what the SKAO will deliver," says Dr. Spekkens, co-author of the paper, who is cross-appointed to Queen’s from the Royal Military College of Canada. "Our results are a really nice illustration of the tremendous value of mapping the sky more deeply and more widely than has ever been done before. This is serendipity at its best."