Scientists at the North Carolina Museum of Natural Sciences and the University of Minnesota Duluth have found a Hoag-type galaxy approximately 359 million light-years away from Earth. The galaxy is described as having a well-defined elliptical-like core surrounded by two circular rings. This class of galaxy is rarely observed and this specific one doesn’t look quite like anything astronomers have observed before. The astronomers named the galaxy PGC 1000714.
Burcin Mutlu-Pakdil, lead author of a paper on this work and a graduate student at the University of Minnesota Duluth, the Minnesota Institute for Astrophysics and the University of Minnesota, Twin Cities explained that less than 0.1% of all observed galaxies are Hoag-type galaxies. Most of the galaxies observed are disc-shaped like our own Milky Way. Hoag-type galaxies consist of a circular ring around a round core, with nothing visibly connecting the two. Astronomers gain unique insights into how galaxies are formed and change when they study galaxies with unusual appearances.
The ages of the two main features of the galaxy, the outer ring and the central body, was determined by collecting multi-waveband images of the galaxy. This can only be done easily in the Southern Hemisphere by using a large diameter telescope in the Chilean mountains.
While the discovery of an older (5.5 billion years) red central core and a younger (0.13 billion years) blue outer ring surrounding it was expected, researchers were surprised to find evidence of a second, obscured inner ring around the central body. Researchers had to take their images and subtract out a model of the core to document this second ring. This allowed them to observe and measure the inner structure of the second ring.
Patrick Treuthardt, co-author of the study and an astrophysicist at the North Carolina Museum of Natural Sciences, added that galaxies with a blue ring around a central red body have been observed before. Hoag’s object is the best known of these. The unique feature of this galaxy is what appears to be an older, diffused red inner ring.
Galaxy rings are areas where stars have been formed from gas colliding. Mutlu-Pakdil notes that this galaxy has probable experienced two different formation periods as indicated by the different colors of the inner and outer ring. It is however not possible to know how the rings of this particular galaxy were formed from the initial single snapshots in time. The researchers believe that astronomers can begin to understand how unusual galaxies are formed and evolve by collecting snapshot views of other galaxies like this one.
The authors guess that the outer ring may well be the result of this galaxy combining with portions of a gas-rich dwarf galaxy that was once nearby. It is however also possible that galaxy shapes are the product of internal or external environmental interactions. The collection of higher resolution infrared data would however be required to deduce the history of the older inner ring.
Treuthardt concludes by saying that we still have a lot to learn and that whenever a unique or strange object is found to study, it challenges our current assumptions and theories about how the universe works.
The full study was published in Monthly Notices of the Royal Astronomical Society.