"This was a serendipitous discovery," said Imad Pasha, the lead researcher and a doctoral student at Yale University in New Haven, Connecticut. "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. I had to stop to investigate it." The team soon began referring to this striking object as the "Bullseye."
Hubble and Keck's follow-up data identified the galaxy that pierced the Bullseye's center: a diminutive blue dwarf situated to the Bullseye's center-left. Around 50 million years ago, this small galaxy darted through the larger one's heart, creating ring-like ripples. Although the pair is currently separated by about 130,000 light-years, a thin stream of gas still links them.
"We're catching the Bullseye at a very special moment in time," said Pieter G. van Dokkum, a co-author of the new study and a professor at Yale. "There's a very narrow window after the impact when a galaxy like this would have so many rings."
Although galaxies commonly collide or nearly miss one another over cosmic timescales, a direct plunge through the core is exceptionally rare. The blue dwarf's trajectory through the Bullseye prompted inward and outward waves of activity, igniting new star formation that gave rise to the Bullseye's multiple rings.
Just how massive is the Bullseye? The Milky Way measures roughly 100,000 light-years across, whereas the Bullseye spans an impressive 250,000 light-years, making it more than twice as large as our home galaxy.
By harnessing Hubble's sharp imaging, the researchers pinpointed most of the Bullseye's rings, many of which overlap near its core. "This would have been impossible without Hubble," Pasha said.
Additional observations from the Keck Observatory affirmed the existence of a ninth ring. The team also suspects a tenth ring may have once existed but has since faded beyond detection, possibly extending three times farther out than the largest ring recorded by Hubble.
"That theory was developed for the day that someone saw so many rings," van Dokkum said. "It is immensely gratifying to confirm this long-standing prediction with the Bullseye galaxy."
From a top-down viewpoint, the rings would appear less evenly spaced than a typical target board. Because Hubble views the galaxy at a slight angle, the pattern looks somewhat compressed. "If we were to look down at the galaxy directly, the rings would look circular, with rings bunched up at the center and gradually becoming more spaced out the farther out they are," Pasha explained.
To visualize how these structures came to be, imagine dropping a pebble into a pond. The first ripple widens over time, creating the largest ring, while each subsequent ring appears after the previous one, collectively forming a set of concentric ripples.
The team suggests the Bullseye's initial pair of rings expanded quickly to form broader outer circles. New rings likely emerged in a staggered way, since the dwarf galaxy's direct pass most strongly affected those first waves of material.
Although individual stars' orbits remained relatively steady, clusters of stars did "pile up" to form recognizable rings over millions of years. Meanwhile, gas was pushed outward and mingled with dust, fueling additional bursts of star creation that brightened each ring.
There's still much to investigate about which stars formed prior to or following the blue dwarf's "fly through." Astronomers will also refine models that reveal how the galaxy might continue evolving over billions of years, including the possible fading of more rings.
Although this ring-laden system emerged serendipitously, researchers expect to find similar galaxies in the near future. "Once NASA's Nancy Grace Roman Space Telescope begins science operations, interesting objects will pop out much more easily," van Dokkum explained. "We will learn how rare these spectacular events really are."
Research Report:The Bullseye: HST, Keck/KCWI, and Dragonfly Characterization of a Giant Nine-ringed Galaxy
Related Links
Hubble Space Telescope
Stellar Chemistry, The Universe And All Within It
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