Maunakea, Hawaiʻi – Astronomers have captured a bizarre image of a supernova, the powerful explosion of a star, whose light was so warped by the gravity of a galaxy that it appears as multiple images in the sky. This effect, known as gravitational lensing, occurs when the gravity of a dense object distorts and brightens the light of an object behind it.
A team led by Ariel Goobar from the Oskar Klein Centre at Stockholm University discovered that the unusual Type Ia supernova dubbed “SN Zwicky” was quadruply-lensed, meaning that four images of the same supernova could be seen from Earth.
The results, which include observations from W. M. Keck Observatory on Maunakea, Hawaiʻi Island, are published in today’s issue of Nature Astronomy.
Within weeks of detecting the supernova at the Zwicky Transient Facility (ZTF) at Palomar Observatory, Goobar and his team used Keck Observatory’s Near-Infrared Camera 2 (NIRC2) paired with its adaptive optics system and successfully resolved SN Zwicky, revealing that the lensing of the supernova was strong enough to have created multiple images of the same object.
“I was observing that night and was absolutely stunned when I saw the lensed image of SN Zwicky,” says Christoffer Fremling, a staff astronomer at the Caltech Optical Observatory who leads the ZTF supernova survey, called the Bright Transient Survey. “We catch and classify thousands of transients with the Bright Transient Survey, and that gives us a unique ability to find very rare phenomena such as SN Zwicky.”
Zooming in to Supernova Zwicky: This video starts with a small portion of an image of the sky captured with the Zwicky Transient Facility camera at Palomar Observatory. This section of the image is one out of 64 “quadrants,” with each quadrant containing tens of thousands of stars and galaxies. The zoom-in then shows larger and sharper observations using the VLT in Chile and W. M. Keck Observatory on Maunakea in Hawai’i. The best resolved Keck Observatory images reveal the four nearly identical “copies” of SN Zwicky. These multiple images arise due to the warping of space caused by a foreground galaxy, also seen in the center and approximately half-way between the site of the supernova explosion and Earth. Credit: J. Johansson
“With ZTF, we have the unique ability to catch and classify supernovae in near real time. We noticed that SN Zwicky was brighter than it should have been given its distance to us and quickly realized that we were seeing a very rare phenomenon called strong gravitational lensing,” says Goobar, lead author of the study and the director of the Oskar Klein Centre at Stockholm University. “Such lensed objects can help us to uniquely probe the amount and distribution of matter at the inner core of galaxies.”
The Very Large Telescope, NASA’s Hubble Space Telescope, Hobby-Eberly Telescope, Liverpool Telescope, and the Nordic Optical Telescope were also used in this study.
As predicted by Albert Einstein more than a century ago, light from one cosmic object that encounters a dense object on its way to us can undergo gravitational lensing. The dense object acts like a lens that can bend and focus the light. Depending on how dense the lens is and the distance between the lens and us, this warping effect can vary in strength. With strong lensing, the light from the cosmic object is so distorted that it is magnified and split into several copies of the same image.
Astronomers have been observing the gravitational bending of light since 1919, just a few years after Einstein developed the theory, but the transient nature of supernovae makes events such as SN Zwicky, also known as SN 2022qmx, very hard to spot. In fact, while scientists have spotted lensed duplicated images of distant objects called quasars many times before, only a handful of supernovae lensed into duplicated images have been found. One classic example, called iPTF16geu, was discovered by the intermediate Palomar Transient Factory (iPTF), a predecessor to ZTF.
“SN Zwicky is the smallest resolved gravitational lens system found with optical telescopes. iPTF16geu was a wider system but had larger magnification,” says Goobar.
A video showing how a galaxy in front of SN Zwicky acted as a magnifying glass that projected four images of the same explosion in Earth’s sky. Credit: ESA/Hubble, L. Calçada. Adapted by J. Johansson
SN Zwicky is what is known as a Type Ia supernova. These are dying stars that end their lives with a light show that is always the same in brightness from event to event. This unique property was used to reveal the accelerated expansion of our universe back in 1998 due to an as-yet unknown phenomenon called dark energy.
“Strongly lensed Type Ia supernovae allow us to see further back in time because they are magnified. Observing more of them will give us an unprecedented chance to explore the nature of dark energy,“ says Joel Johansson, a postdoctoral fellow at Stockholm University and a co-author of the study.
“What are missing components needed to model the expansion history of the universe? What is the dark matter that makes up the vast majority of the mass in galaxies? As we discover more ‘SN Zwickys’ with ZTF and the upcoming Vera Rubin Observatory, we will have another tool to chip away at the mysteries of the universe and find answers,” says Goobar.
The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
ABOUT ADAPTIVE OPTICS
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and current systems now deliver images three to four times sharper than the Hubble Space Telescope at near-infrared wavelengths. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck 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.