A Type Ia supernova named Encore is the second multiply-imaged supernova detected in MRG-M0138, a galaxy around 10 billion light-years away in the constellation of Cetus.
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These NIRCam/Webb images show the multiply-imaged supernova Encore in the distant galaxy MRG-M0138. Two images of Encore (circled) are seen here, but an additional supernova image is expected to become visible around 2035. Image credit: NASA / ESA / CSA / STScI / Justin Pierel, STScI / Andrew Newman, Carnegie Institution for Science.
In November 2023, astronomers using the NASA/ESA/CSA James Webb Space Telescope observed a massive cluster of galaxies called MACS J0138.0-2155.
Through the gravitational lensing effect, the MRG-M0138 galaxy appears warped by the powerful gravity of the intervening galaxy cluster.
In addition to warping and magnifying the distant galaxy, the gravitational lensing caused by MACS J0138.0-2155 produces five different images of MRG-M0138.
In 2019, astronomers announced the surprising discovery of a supernova within MRG-M0138, as seen in images from the NASA/ESA Hubble Space Telescope taken in 2016.
When another group of astronomers examined the newest Webb images, they were astonished to find that seven years later, the same galaxy is home to a second supernova.
“When a supernova explodes behind a gravitational lens, its light reaches Earth by several different paths,” said Dr. Justin Pierel from the Space Telescope Science Institute and Dr. Andrew Newman from the Observatories of the Carnegie Institution for Science.
“We can compare these paths to several trains that leave a station at the same time, all traveling at the same speed and bound for the same location.”
“Each train takes a different route, and because of the differences in trip length and terrain, the trains do not arrive at their destination at the same time.”
“Similarly, gravitationally lensed supernova images appear to astronomers over days, weeks, or even years.”
“By measuring differences in the times that the supernova images appear, we can measure the history of the expansion rate of the Universe, known as the Hubble constant, which is a major challenge in cosmology today.”
“The catch is that these multiply-imaged supernovae are extremely rare: fewer than a dozen have been detected until now.”
“Within this small club, the 2016 supernova in MRG-M0138, named Requiem, stood out for several reasons,” they explained.
“First, it was 10 billion light-years distant. Second, the supernova was likely the same type (Type Ia) that is used as a ‘standard candle’ to measure cosmic distances. Third, models predicted that one of the supernova images is so delayed by its path through the extreme gravity of the cluster that it will not appear to us until the mid-2030s.”
“Unfortunately, since Requiem was not discovered until 2019, long after it had faded from view, it was not possible to gather sufficient data to measure the Hubble constant then.”
“Now we have found a second gravitationally lensed supernova within the same galaxy as Requiem, which we call Encore.”
“Encore was discovered serendipitously, and we are now actively following the ongoing supernova with a time-critical director’s discretionary program.”
“Using these Webb images, we will measure and confirm the Hubble constant based on this multiply imaged supernova.”
“Encore is confirmed to be a standard candle or Type Ia supernova, making Encore and Requiem by far the most distant pair of standard-candle supernova ‘siblings’ ever discovered.”
“Supernovae are normally unpredictable, but in this case we know when and where to look to see the final appearances of Requiem and Encore.”
“Infrared observations around 2035 will catch their last hurrah and deliver a new and precise measurement of the Hubble constant.”
