RING IT ON: W. M. KECK OBSERVATORY DATA HITS A GALACTIC BULLSEYE Shot through the heart – and far smaller blue dwarf galaxy is to blame

RING IT ON: W. M. KECK OBSERVATORY DATA HITS A GALACTIC BULLSEYE
Shot through the heart – and far smaller blue dwarf galaxy is to blame
Maunakea, Hawaiʻi – W. M. Keck Observatory on Maunakea, Hawaiʻi Island and NASA’s Hubble Space Telescope have captured a cosmic bullseye! The gargantuan galaxy LEDA 1313424 is rippling with nine star-filled rings after an “arrow” — a far smaller blue dwarf galaxy — shot through its heart.
Astronomers using Hubble identified eight visible rings, more than previously detected by any telescope in any galaxy and confirmed a ninth ring using data from the Keck Cosmic Web Imager (KCWI) at Keck Observatory. Previous observations of other galaxies show a maximum of two or three rings.
The study, led by Yale University, is published in today’s issue of The Astrophysical Journal Letters.

“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 later nicknamed the galaxy the “Bullseye.”
Keck Observatory and Hubble’s follow-up observations helped the researchers prove which galaxy plunged through the center of the Bullseye — a blue dwarf galaxy to its center-left. This relatively tiny interloper traveled like a dart through the core of the Bullseye about 50 million years ago, leaving rings in its wake like ripples in a pond. A thin trail of gas now links the pair, though they are currently separated by 130,000 light-years.
“We’re catching the Bullseye at a very special moment in time,” said Pieter G. van Dokkum, a co-author and a professor at Yale. “There’s a very narrow window after the impact when a galaxy like this would have so many rings.”
Galaxies collide or barely miss one another quite frequently on cosmic timescales, but it is extremely rare for one galaxy to dive through the center of another. The blue dwarf galaxy’s straight trajectory through the Bullseye later caused material to move both inward and outward in waves, setting off new regions of star formation.
“KCWI provided the critical view of this companion galaxy that we see in projection near the bullseye,” said Pasha. “We found a clear signature of gas extending between the two systems, which allowed us to confirm that this galaxy is in fact the one that flew through the center and produced these rings.”
How big is the Bullseye? Our Milky Way galaxy is about 100,000 light-years in diameter, and the Bullseye is almost two-and-a-half times larger, at 250,000 light-years across.
The researchers used Hubble’s crisp vision to carefully pinpoint the location of most of its rings, since many are piled up at the center, and used Keck Observatory’s KCWI to confirm the existence of the ninth ring. The team suspects a tenth ring also existed but has faded and is no longer detectable. They estimate it might lie three times farther out than the widest ring in Hubble’s image.
There’s a lot more research to be done to figure out which stars existed before and after the blue dwarf’s “fly through.” Astronomers will now also be able to improve models showing how the galaxy may continue to evolve over billions of years, including the disappearance of additional rings.
“The data from KCWI that identified the ‘dart’ or impactor is unique,” van Dokkum said. “There hasn’t been any other case where you can so clearly see the gas streaming from one galaxy to the other. That there is all this gas right between the velocity of one galaxy and the other is the key insight, showing that material is being pulled out of one galaxy, left behind by the other, or both. It physically fills up the entire space. The KCWI data enables us to see the tendril of gas that is still connecting these two galaxies.”

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ABOUT KCWI
The Keck Cosmic Web Imager (KCWI) is designed to provide visible band, integral field spectroscopy with moderate to high spectral resolution formats and excellent sky-subtraction. The astronomical seeing and large aperture of the telescope enables studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters, and lensed galaxies. KCWI covers the blue side of the visible spectrum; the instrument also features the Keck Cosmic Reionization Mapper (KCRM), extending KCWI’s coverage to the red side of the visible spectrum. The combination of KCWI-blue and KCRM provides simultaneous high-efficiency spectral coverage across the entire visible spectrum. Support for KCWI was provided by the National Science Foundation, Heising-Simons Foundation, and Mt. Cuba Astronomical Foundation. Support for KCRM was provided by the National Science Foundation and Mt. Cuba Astronomical Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. For more information, visit:www.keckobservatory.org