The Nine-Ringed, Record-Breaking Bullseye Galaxy – Sky & Telescope

The Hubble Space Telescope has revealed a celestial rarity: a galaxy decorated with an unprecedented nine concentric rings. Likened to ripples in a pond, the rings of the newly discovered LEDA 1313424 owe their configuration to a much smaller blue dwarf galaxy that passed through the larger galaxy’s core 50 million years ago. The dwarf now lies 130,000 light-years away; a tenuous trail of gas connects the two.

The ring galaxy, nicknamed “Bullseye,” is located 567 million light-years away in the constellation Pisces. It is 250,000 light-years wide, about 2½ times the span of the Milky Way. Bullseye surprised scientists with its many star-filled rings, as previously observed collisional ring galaxies feature a maximum of two or three rings. The Yale-led team found eight rings in Hubble data and a ninth ring in data from the Keck Observatory in Hawai‘i; they also used data from the Dragonfly Telephoto Array, which is designed to image objects with extremely faint surface brightness. The images appear in the February 10th Astrophysical Journal Letters.

“This was a serendipitous discovery,” explains lead researcher Imad Pasha (Yale University). “I was looking at a ground-based imaging survey and when I saw a galaxy with several clear rings, I was immediately drawn to it.”

The collision that produced the rings occurred in just the right way to gravitationally warp the giant galaxy into its concentric configuration; it’s exceptionally uncommon for a colliding galaxy to pass through its neighbor’s center in such a straight trajectory.

The fly-through sent material inward and outward, triggering mass star birth as dust and gas crashed together, brightening the expanding rings. The astronomers were also fortunate that Bullseye’s orientation allowed a full view of its rings.

Timing is equally essential. “We’re catching the Bullseye at a very special moment,” the “very narrow” period when it still has (almost) all of its rings, says team member Pieter G. van Dokkum (also at Yale). Ring systems, whether made of gas and stars or rocks and ice, don’t last long in cosmic terms. Yet we occupy the right place and post-collisional timeframe to glimpse the short-lived structures.

The Hubble imaging shows that many of the inner rings are still piled up near the galactic center, while the outer rings extend on scales of tens of thousands of light-years.

At the galaxy’s outermost edges, ground-based imaging via the Dragonfly Telephoto Array revealed faint patches that might be associated with an older, dissipated ring, with a radius of approximately 230,000 light-years.

The data on Bullseye match a long-standing theory published more than a decade ago, which predicted that the rings’ expand outward over time. That theory even predicts the existence of the 10th ring. Further research, such as discerning stars that were already there before the collision from those that formed in post-collisional nurseries, will help improve that model to better forecast Bullseye’s future evolution.

Bullseye hosts a lot of neutral hydrogen gas, considering its mass in stars. That reservoir of star-forming material is similar to known low surface brightness galaxies, strengthening the notion that collisional ring galaxies evolve into these fainter objects as their rings fade. Accordingly, researchers cite this as the first observational evidence for this galactic evolutionary pathway.

The Milky Way demonstrates evidence of a similar albeit more minor clash with a dwarf galaxy, not in rings but in ripples. Previous observations have shown that the Milky Way’s disk undulates like corrugated cardboard, features that could be due to a long-ago collision with a dwarf galaxy.

If that scenario proved true, Pasha says, “it would be accurate to say the Bullseye formed under a similar qualitative scenario, but with very different parameters, leading to a dramatically different outcome.” He explains that while the Milky Way survived with only little swells superimposed on its spiral arms, the massive, central impact that the Bullseye experienced “dramatically reconfigured” that galaxy.

The timing of the Bullseye discovery is pertinent in terrestrial terms: The announcement of a celestial outlier born of an arrow piercing a galactic heart is fitting as Valentine’s Day draws nigh. But this serendipitous find will multiply once NASA’s Nancy Grace Roman Telescope launches later this decade, as “interesting objects will pop out much more easily,” says van Dokkum.