Looking at a galaxy about 359 million light years away, Burcin Mutlu-Pakdil was unsure that what she saw was real.
She was researching a rare kind of galaxy called a Hoag-type galaxy. Their round cores surrounded by a circular ring differentiate them from the majority of galaxies, which are disc-shaped, including the Milky Way. Fewer than 0.1 percent of galaxies are Hoag-type, but this one became even more rare when Mutlu-Pakdil said she saw a second ring.
“When our study revealed the inner ring, we were so surprised that I checked my studies, analyzed it several times. Am I making a mistake? Is this really true that this is correct? Then after several times, and every independent metrics showed the same thing, then I was like, ‘OK, this is so unique.’ This was so amazing for me,” said Mutlu-Pakdil, a graduate student at the University of Minnesota Duluth.
She is the lead author of a paper published last week in the Monthly Notices of the Royal Astronomical Society, which provides the first description of the galaxy, named PGC 100714 because it hasn’t been given a common name yet.
Co-author Marc Seigar, associate dean of UMD’s Swenson College of Science and Engineering, said the galaxy is unique because of its second ring.
“We don’t understand how it got the inner ring, but that makes it even rarer than Hoag-type galaxies. No one’s ever seen a galaxy like this,” Seigar said.
The galaxy is easily observable only in the Southern Hemisphere. Seigar wrapped up a tour observing 600 galaxies in the Southern Hemisphere a decade ago. Patrick Treuthardt, another co-author now working at the North Carolina Museum of Natural Sciences, was working for Seigar as a postdoctoral student at the University of Arkansas when he noticed PGC 100714 while looking at the images of the galaxies Seigar had observed from the Chilean mountains.
“It just happened to be in the background of an image of another galaxy that I was interested in at the time,” Seigar said.
After Seigar moved to UMD, he said he had several students take a look at it in more detail. Mutlu-Pakdil, who is graduating with her doctorate in May, studied the galaxy for about eight months as part of her graduate work.
The galaxy has a blue outer ring that’s 0.13 billion years old and a red core that’s 5.5 billion years old. Galaxy rings are regions where stars have formed from colliding gas, and the different colors suggest that the galaxy had two different formation periods. However, the formation and nature of the galaxy are still unknown, Mutlu-Pakdil said. The discovery is exciting because the galaxy is an “extreme case,” she added.
“Understanding those extreme cases will really improve our understanding of galaxy formation, so they are important because of that,” she said. “This really shows us that we really have lots of things to learn and investigate and, because of that, I’m really excited to be a part of this puzzle.”
What’s unknown is the interesting part of the research on this particular galaxy, Seigar said.
“I think that because it’s so unique, it really stretches the boundaries of our understanding of how galaxies form and evolve. When we find these outlying objects, our theories have to be able to explain them as well as normal galaxies. It adds to the wealth of information that we need to explain with our theories,” Seigar said.
Mutlu-Pakdil said she wants to study the galaxy more to really understand how it formed, but she’s applying to postdoctoral positions to begin after graduation, so she’ll wait to see how things settle.
UMD’s Swenson college paid for the research, and the Theodore Dunham Jr. Fund for Astrophysical Research provided computational equipment.