Astronomers have discovered that a nearby double-galaxy system has a giant, star-studded ring.

ring galaxy ESO 179-13
Astronomers have discovered a ring of star-forming knots around a dwarf spiral galaxy, created where another galaxy (upper left) tore through the spiral. This color composite of the collision is a combo of a hydrogen-alpha image and images taken at red and blue wavelengths.
Credit: Ivan Bojicic / Q. A. Parker et al.

Observers first discovered the galaxy pair ESO 179-13 back in 1974. It comprises a dwarf spiral galaxy and a smaller, ragged galaxy just to its northeast in the constellation Ara, deep in the Southern Hemisphere sky. (Another team found a third, diminutive component in 2001, but it’s superfluous to this tale.)

ESO 179-13 hasn’t received much attention over the decades because it’s inconveniently located in a crowded patch of sky behind our Milky Way Galaxy’s disk. Quentin Parker (University of Hong Kong and Australian Astronomical Observatory) and colleagues have now taken a careful look at the system, using a combination of images and spectra taken at various wavelengths. What they found is a big ring of knotty, star-forming hydrogen gas around the dwarf spiral, perhaps stripped out of that galaxy when its smaller companion shot through it like a bullet several tens of millions of years ago.

Astronomers know of less than 20 systems like ESO 179-13 that are convincingly cartwheel-type galaxies — a name taken from the iconic Cartwheel Galaxy (ESO 350-40). At only 30-ish million light-years away, ESO 179-13 is the closest yet found. It’s also one of the smallest: the ring is only 20,000 light-years across, and the spiral galaxy (almost perfectly) at its center is mass-wise on par with the Magellanic Clouds, dwarf galaxies currently making a pass by the Milky Way. That’s neat because it shows that dwarfs — which are ballpark only a hundredth as massive as galaxies like our own — can also collide and create bull’s-eye systems.

You can read more about the discovery in the press release from the Royal Astronomical Society, or in the team’s paper, which appears in the October 1st Monthly Notices of the Royal Astronomical Society.


Reference: Q. A. Parker et al. “Kathryn’s Wheel: A Spectacular Galaxy Collision Discovered in the Galactic Neighbourhood.” Monthly Notices of the Royal Astronomical Society. October 1, 2015.

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