Large clusters of galaxies move through space-time to such an extent that they act like a lens. The light of more distant galaxies is distorted and magnified, giving astronomers the opportunity to see more detail than is normally possible. The case of the galaxy MRG-M0138 is particularly exciting, because it was seen by Hubble with a supernova explosion, which they call Requiem. Today, it has been announced that JWST has seen a new supernova in November 2023 which is exciting news not only for observations of supernovae, but because it will help us understand the expansion of the universe.
The cluster of galaxies in front of MRG-M0138 creates five different images of the galaxy, meaning five different views of the supernova. Light takes different paths, and on one path, it takes longer to reach us; it is estimated that this image of the explosion will not be visible to us until the 2030s.
When a supernova explodes behind a gravitational lens, its light reaches Earth through many different paths. We can compare these routes to many trains leaving a station at the same time, all traveling at the same speed and to the same location. Each train takes a different route, and because of the difference in travel time and terrain, the trains will not reach their destination at the same time, Justin Pierel, from the Space Telescope Science Institute, and Andrew Newman, from the Carnegie Institution for Science, wrote in a statement.
Similarly, gravitationally lensed supernova images can be seen by astronomers for days, weeks, or even years. By measuring the differences in the times when supernova images are visible, 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 rare: less than a dozen have been found so far.
Requiem is a Type Ia supernova. These events are created by a white dwarf stealing enough material from a companion that it crosses a mass threshold and collapses under its own weight. Since the threshold is the same, the supernova reaches the same brightness as a standard candle. How bright an object is depends on its intrinsic brightness and its distance. We can measure the brightness and the brightness is given for standard candles, so we can use both measurements to check the distance of the galaxies. From there, we get to other cosmological properties.
Location and scene of Requiem on the left, and Encore on the right.
Hubble image credit: NASA, ESA, STScI, Steve A. Rodney (University of South Carolina) and Gabriel Brammer (Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen); JWST image credit: NASA, ESA, CSA, STScI, Justin Pierel (STScI) and Andrew Newman (Carnegie Institution for Science).
Hubble was too late in Requiem to make detailed calculations from the four images, so the work will be done in the next decade on the fifth. But JWST caught the event in the act, and the team was given more time to quickly follow the event.
Now we have found a second gravitationally lensed supernova within the same galaxy as Requiem, which we call Supernova Encore. Encore has been discovered for nothing, and we are now actively following the ongoing supernova with a critical time directors discretionary program, the duo continued.
Encore is also a Type Ia supernova, making it a serendipitous and lucky observation. Data collection is ongoing, so we will know more in the coming months. And of course, there will be an encore in the 2030s.
Supernovas are often unpredictable, but in this case we know when and where to look to see the final manifestation of Requiem and Encore. Infrared observations around 2035 will catch their last hurray and provide a new and accurate measurement of the Hubble constant, the researchers concluded.
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