Ring nebulae and the rings of Saturn are among some familiar rings that exist in space. But consider ring galaxies!
Trillions of galaxies populate the universe. But one of the most fascinating and beautiful types of galaxies is not commonly known to many people. We hear a lot about spiral, elliptical, irregular, dwarf, and even ultra-diffuse galaxies, all with secrets yet to be revealed. But what about ring galaxies? Only 1-in-10,000 galaxies fall into this rarest category of all.
The first ring galaxy was discovered in 1950. It is named for its discoverer, Arthur Hoag. He first identified it as either a planetary nebula or a “peculiar” galaxy. Known as Hoag’s Object, it lies about 613 million light years away towards the constellation of Serpens Caput, the
head of the serpent. Hoag’s Object spans approximately 100,000 light years, slightly larger than our own Milky Way galaxy.
About 40,000 ring galaxies have been discovered by astronomers using a deep learning algorithm and data from the Galaxy Zoo science project. They account for less than 0.1% of all known galaxies!
In one respect they have a somewhat common galactic structure, with a dense core consisting mostly of old reddish-yellow stars, and an outer circular ring consisting of clusters of bright blue, massive, young stars.
Hoag’s Object has a gap in between the core and the outer ring which may be home to faint star clusters.
Studies of galaxy formation have uncovered many surprises such as black holes at their centers and the inferred existence of halos of so called “dark matter”. But how ring galaxies were formed remains a mystery. Some current theories suggest that Hoag-type galaxies form when two galaxies collide and the typical spiral arm structure is gravitationally disrupted. A ring is thought to form when a smaller galaxy passes through the central part of a spiral galaxy at a right angle to the spiral's disk, and the impact ripples outward. Gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy.
Other astronomers think that rings are formed around some galaxies when external accretion takes place. Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material.
In another theory, bar instability in barred spiral galaxies could lead to the formation of a ring structure rather than a spiral structure.
What is the true mechanism of ring galaxy formation? As with so many other mysteries in this vast universe, the answer is “we don’t know - yet”. Stay tuned.
Treat yourself – go online and find pictures of Hoag’s Object. Visible in the gap between the core and the outer ring one can see a far more distant, second ring galaxy in the photo! Now, what are the odds of that! I say, they’re astronomical!
You’ve been listening to Western Slope Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and recorded by Nancy McGuire.
