What happens when one galaxy shoots a bigger galaxy right through the heart?
Like a rock thrown into a pond, the smashup creates a splash-up of starry ripples.
At least that’s what happened to the Bullseye galaxy, which is the focus of observations made by NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii.
In a study published today by The Astrophysical Journal Letters, a research team led by Yale University’s Imad Pasha identifies nine visible ring-shaped ripples in the structure of the galaxy, formally known as LEDA 1313424. The galaxy is 567 million light years from Earth in the constellation Pisces.
The Bullseye now holds the record for the most rings observed in a galaxy. Previous observations of other galaxies showed a maximum of two or three rings.
“This was a serendipitous discovery,” Pasha said in a news release. “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.”
Eight separate rings could be spotted in the image captured by Hubble’s Advanced Camera for Surveys.
The ninth ring was identified in data from the Keck Observatory. Follow-up observations also helped the team figure out which galaxy plunged through the Bullseye’s core.
It’s the blue dwarf galaxy visible to the center-left of LEDA 1313424 in the Hubble image.
Researchers say the current view captures the state of the Bullseye about 50 million years after the blue dwarf blasted through its core. Even though the two galaxies are separated by 130,000 light-years, a thin trail of gas still links them together.
“We’re catching the Bullseye at a very special moment in time,” said Yale Professor Pieter G. van Dokkum, a study co-author. “There’s a very narrow window after the impact when a galaxy like this would have so many rings.”
The multi-ringed shape conforms to the mathematical models for a headlong galaxy-on-galaxy collision. The blue dwarf’s impact caused galactic material to move both inward and outward, sparking multiple waves of star formation along the lines of the ripples — almost exactly as the models predicted.
“It is immensely gratifying to confirm this longstanding prediction with the Bullseye galaxy,” van Dokkum said.
The models suggest that the first two rings in the Bullseye formed quickly and spread out in wider circles. The timing for the formation of additional rings was staggered as the blue dwarf plowed through the bigger galaxy’s core.
The research team suspects that there was once a 10th ring to the galaxy, but that it faced out and is no longer detectable. That ring might have been as much as three times farther out than the widest ring seen in the Hubble image.
Compared to our own Milky Way galaxy, the Bullseye is a big target. It’s about 250,000 light-years wide, as opposed to 100,000 light-years for the Milky Way.
Billions of years from now, the Milky Way and the neighboring Andromeda galaxy are due to collide, but computer simulations suggest that the dynamics of that collision will be more complex than merely dropping a cosmic rock into a pond, or shooting an arrow through a bull’s-eye.
Fortunately, astronomers won’t have to wait billions of years to see more spot-on galactic collisions. “Once NASA’s Nancy Grace Roman Space Telescope begins science operations, interesting objects will pop out much more easily,” van Dokkum said. “We will learn how rare these spectacular events really are.”
In addition to Pasha and van Dokkum, the authors of the Astrophysical Journal Letters study, “The Bullseye: HST, Keck/KCWI, and Dragonfly Characterization of a Giant Nine-Ringed Galaxy,” include Qing Liu, William P. Bowman, Steven R. Janssens, Michael A. Keim, Chloe Neufeld and Roberto Abraham.
Written by Alan Boyle/Universe Today.
