An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes the discovery of a galaxy of a very rare type, with a central core surrounded by a pair of rings. Called PGC 1000714, it was discovered by a group of researchers at the University of Minnesota Duluth and the North Carolina Museum of Natural Sciences who recognized it as similar to the Hoag’s object, a ring galaxy.
The galaxy PGC 1000714 is about 359 million light years from Earth and after its discovery was studied at different frequencies of the electromagnetic spectrum to try to better understand its characteristics. Less than a galaxy in a thousand is the so-called Hoag type, a category called that way after Arthur Allen Hoag in 1950 discovered the first of that type, which he classified as a planetary nebula or a galaxy of unusual shape. Later it received the designations PGC 54559 PRC and D-51 but is commonly called Hoag’s object while its class is also called a ring galaxy.
In the case of the galaxy PGC 1000714, multispectral images allowed to estimate the age of the core and the ring but this work reserved a new surprise with the discovery of a second inner ring around the core. The age of the central and older red core was estimated at around 5.5 billion years while the outer blue and young ring was estimated at 130 million years. The inner ring is red and old, like the core.
The image shows on the left a false-color picture of the galaxy PGC 1000714 where you can see very well its outer ring and its core. On the right there is a galaxy’s B-I color index map where the outer ring is visible in blue while the inner ring is more diffuse and in light green color.
The presence of two such different age structures in the galaxy PGC 1000714 suggests that it passed through two evolutionary periods. The outer ring might have been created by incorporating parts of a gas rich dwarf galaxy that was once very close in what is called an accretion event.
To get an idea of the inner ring’s origin the researchers will try to obtain infrared images at a higher resolution. They believe that they need to accumulate snapshots of other ring galaxies over time to understand the mechanisms of their formation and evolution, which at present are not yet well understood.