Webb Sees a Supernova Go Off in a Gravitationally Lensed Galaxy – for the Second Time

Nature, in its infinite inventiveness, provides natural astronomical lenses that allow us to see objects beyond the normal reach of our telescopes. They’re called gravitational lenses, and a few years ago, the Hubble Space Telescope took advantage of one of them to spot a supernova explosion in a distant galaxy.

Now, the JWST has taken advantage of the same lens and found another supernova in the same galaxy.

A gravitational lens is a massive object like a galaxy or galaxy cluster. The object’s mass creates a curvature in space-time. When light from an object behind the lens travels past the cluster, it’s magnified. Most gravitational lenses were discovered accidentally, but recently, dedicated searches have found more of them, and they’ve become an important tool in astronomy.

In 2019, astronomers found a supernova in images the Hubble Space Telescope captured in 2016 of a galaxy named MRG-M0138. Those images were gravitationally lensed by a galaxy cluster called MACS J0138.0-2155.

Now, the JWST has observed the same galaxy through the same lens and found another supernova that exploded only seven years after the previous one. This is remarkable and is the first time astronomers have found two supernovae in the same galaxy.

Gravitational lenses do more than just magnify background objects. They also create multiple images of the objects. But the images don’t arrive at the same time, and their temporal separation is another astronomical tool.

In a NASA blog post, Justin Pierel of NASA’s Space Telescope Science Institute (STScI) and Andrew Newman from the Observatories of the Carnegie Institution for Science explained the findings.

“When a supernova explodes behind a gravitational lens, its light reaches Earth by several different paths,” the pair explain. “We can compare these paths to several trains that leave a station at the same time, all travelling 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.”

So, images of a single supernova can arrive at our telescopes at different times, sometimes separated by several years. This arrangement, though it can seem confounding, is actually another useful tool. Studying the images can help scientists measure the Hubble constant, which is the history of the expansion rate of the Universe. “The catch is that these multiply-imaged supernovae are extremely rare: fewer than a dozen have been detected until now,” the pair of scientists explain.

“Within this small club, the 2016 supernova in MRG-M0138, named Requiem, stood out for several reasons,” Pierel and Newman explain. The first is that the supernova is 10 billion light years away. The second is that it’s also a Type 1a supernova. Type 1a supernovae serve as standard candles, objects with known luminosities that can be used to gauge distances in the cosmic distance ladder. The third reason is that one of the images will be so delayed that it won’t arrive until the middle of the 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,” the scientists explain.

But now Webb has observed a second supernova called Encore.

“Encore was discovered serendipitously, and we are now actively following the ongoing supernova with a time-critical director’s discretionary program,” the scientists write. “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.”

Measuring the Hubble constant is an ongoing challenge in cosmology. The expansion of the Universe is the prime piece of evidence supporting the Big Bang. So getting the constant right is an important part of understanding the Universe. The Hubble constant measures how galaxies are moving away from us at speeds proportional to their distance. It’s expressed in km/s of a galaxy 1 megaparsec away, and over the decades, different researchers have come up with different numbers. The most recent measurement of Hubble’s constant is 68.3 (km/s)/Mpc.

Shortly after the Big Bang, the Universe was expanding due to inflation. About three billion years ago, the mysterious force we’ve named Dark Energy took over. Dark Energy’s force isn’t diluted as the Universe expands, and it’s still driving the expansion. In fact, that expansion is accelerating, and we don’t know why. But somehow, an accurate measurement of the Hubble Constant is part of the explanation. And measuring the same supernova several years apart will yield an accurate measurement.

The quest for the most accurate measurement is the quest for a better understanding of the Universe. Finding two standard candle supernovae in the same galaxy is a unique opportunity to measure the Hubble constant more accurately than ever before. “Considering the rarity of finding multiple SNe Ia in the same host galaxy, compounded with the extreme rarity of lensed SNe, this discovery is truly surprising,” the scientists wrote in their observing proposal.

Supernovae in distant galaxies don’t give us any prior indication that they’re going to explode. We know which type of stars will explode and end themselves as supernovae, but can’t measure when. But this case is unique. We know when the next one will appear, or rather when the next image of the same one will appear.

“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,” the pair of scientists write.