WASHINGTON: Scientists have for the first time observed a rare galaxy with two outer rings surrounding a red core located about 359 million light-years away from Earth.
The galaxy named PGC 1000714 appears to belong to a class of rarely observed, Hoag-type galaxies.
It has a well-defined elliptical-like core surrounded by two circular rings.
"Less than 0.1 per cent of all observed galaxies are Hoag-type galaxies," said Burcin Mutlu-Pakdil, a graduate student at University of Minnesota in the US.
Hoag-type galaxies are round cores surrounded by a circular ring, with nothing visibly connecting them. The majority of observed galaxies are disc-shaped like our own Milky Way. Galaxies with unusual appearances give astronomers unique insights into how galaxies are formed and change.
The researchers collected multi-waveband images of the galaxy, which is only easily observable in the Southern Hemisphere, using a large diameter telescope in the Chilean mountains.
These images were used to determine the ages of the two main features of the galaxy, the outer ring and the central body.
While the researchers found a blue and young (0.13 billion years) outer ring, surrounding a red and older (5.5 billion years) central core, they were surprised to uncover evidence for second inner ring around the central body.
To document this second ring, researchers took their images and subtracted out a model of the core. This allowed them to observe and measure the obscured, second inner ring structure.
"We've observed galaxies with a blue ring around a central red body before, the most well-known of these is Hoag's object. However, the unique feature of this galaxy is what appears to be an older diffuse red inner ring," said Patrick Treuthardt, an astrophysicist at the North Carolina Museum of Natural Sciences.
Galaxy rings are regions where stars have formed from colliding gas.
"The different colors of the inner and outer ring suggest that this galaxy has experienced two different formation periods," Mutlu-Pakdil said.
"From these initial single snapshots in time, it's impossible to know how the rings of this particular galaxy were formed," she said.
The researchers said that by accumulating snapshot views of other galaxies like this one astronomers can begin to understand how unusual galaxies are formed and evolve.
While galaxy shapes can be the product of internal or external environmental interactions, researchers speculate that the outer ring may be the result of this galaxy incorporating portions of a once nearby gas-rich dwarf galaxy.