Astronomers have spotted for the first time an elliptical galaxy with two fairly round rings.
PGC 1000714 (center). Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD / SDSS 9.
The galaxy in question is called PGC 1000714 (also known as 2MASX J11231643−0840067).
It lies in the small constellation Crater, about 372 million light-years away, and belongs to a class of rarely observed, Hoag-type galaxies.
“Hoag-type galaxies, which bear strong resemblance to Hoag’s Object (PGC 054559, Hoag 1950), are round cores surrounded by a circular ring, with nothing visibly connecting them,” explained Burcin Mutlu-Pakdil, a Ph.D. candidate at University of Minnesota Twin Cities.
“These galaxies are extremely rare (less than 0.1% of all observed galaxies) and their origin is still debated.”
“The majority of observed galaxies are disc-shaped like our Milky Way Galaxy,” said Mutlu-Pakdil, lead author of a paper on this work in the Monthly Notices of the Royal Astronomical Society.
The left panel shows a false-color image of PGC 1000714. The right panel shows a B-I color index map that reveals both the outer ring (blue) and diffuse inner ring (light green). Image credit: Ryan Beauchemin.
Mutlu-Pakdil and her colleagues collected multi-waveband images of PGC 1000714, which is only easily observable in the Southern Hemisphere, using the 2.5-m (100-inch) Irénée du Pont telescope at the Las Campanas Observatory in Chile.
The images were used to determine the ages of the two main features of PGC 1000714: the outer ring and the central body.
While the astronomers found a blue and young (0.13 billion year old) outer ring, surrounding a red and older (5.5 billion year old) central core, they were surprised to uncover evidence for second inner ring around the central body.
To document this second ring, they 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,” said co-author Dr. Patrick Treuthardt, a researcher at the North Carolina Museum of Natural Sciences.
“However, the unique feature of PGC 1000714 is what appears to be an older diffuse red inner ring.”
“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.”
“By accumulating snapshot views of other galaxies like this one, astronomers can begin to understand how unusual galaxies are formed and evolve,” the authors said.
“While galaxy shapes can be the product of internal or external environmental interactions, the outer ring may be the result of this galaxy incorporating portions of a once nearby gas-rich dwarf galaxy.”
_____
Burçin Mutlu Pakdil et al. 2017. A photometric study of the peculiar and potentially double ringed, non-barred galaxy: PGC 1000714. MNRAS 466 (1): 355-368; doi: 10.1093/mnras/stw3107
