Astronomers have for the first time spotted four images of a distant exploding star, arranged in a cross-shape pattern by a powerful gravitational lens. In addition to being a unique sighting, the discovery will provide insight into the distribution of dark matter. The findings will appear March 6 in a special issue of the journal Science, celebrating the centenary of Albert Einstein’s Theory of General Relativity.
Two teams spent a week analyzing the object’s light, confirming it was the signature of a supernova, then turned to the W. M. Keck Observatory on Mauna Kea, in Hawaii, to gather critical measurements including determining the distance to the supernova’s host galaxy 9.3 billion light-years from Earth.
To explain the unique, four-up projection, the scientists determined a galaxy cluster and one of its massive elliptical members are gravitationally bending and magnifying the light from the supernova behind it, through an effect called gravitational lensing. First predicted by Albert Einstein, this effect is similar to a glass lens bending light to magnify and distort the image of an object behind it. The multiple images, arranged around the massive elliptical galaxy, form an Einstein Cross, a name originally given to a multiple-lensed quasar that appear as a cross.
Although astronomers have discovered dozens of multiply imaged galaxies and quasars, they have never seen a stellar explosion resolved into several images. “It really threw me for a loop when I spotted the four images surrounding the galaxy – it was a complete surprise,” said Patrick Kelly of the University of California, Berkeley, lead author of the paper and a member of the Grism Lens Amplified Survey from Space (GLASS) collaboration. The GLASS group is working with the FrontierSN team to analyze the supernova.
“This short-lived object was discovered because Pat Kelly very carefully examined the Hubble Space Telescope data and noticed a peculiar pattern,” noted Alex Filippenko, Professor of Astronomy at the University of California, Berkeley, and a member of the team. “Luck comes to those who are prepared to receive it.”
“The LRIS spectrograph on Keck I was used to measure a spectrum at the location of the supernova and was used to measure the distance to the supernova host galaxy,” Tommaso Treu, the GLASS project’s principal investigator and Professor of Physics and Astronomy at the University of California, Los Angeles. “Furthermore, the spectrum was used to determine the intrinsic duration of the event: as a result of the expansion of the universe, distant events appeared stretched in time to us. For example a NBA basketball game in the supernova host galaxy would appear to us to last 120 minutes, instead of the standard 48 minutes it does on Earth. Finally, the non-detection of emission from the supernova itself allowed the team to rule out some potential contaminants and provides clues as to the type of supernova.”
This unique observation will help astronomers refine their estimates of the amount and distribution of dark matter in the lensing galaxy and cluster. Dark matter cannot be seen directly but is believed to make up most of the universe’s mass.
When the four images do fade away, astronomers will have a rare chance to catch a rerun of the supernova. This is because the current four-image pattern is only one component of the lensing display. The supernova may have appeared in a single image some 20 years ago elsewhere in the cluster field, and it is expected to reappear once more in the next one to five years.
The prediction of a future appearance is based on computer models of the cluster, which describe the various paths the divided light is taking through the maze of clumpy dark matter in the galactic grouping. Each image takes a different route through the cluster and arrives at a different time, due, in part, to differences in the length of the pathways the light follows to reach Earth. The four supernova images captured by Hubble, for example, appeared within a few days or weeks of each other.
The supernova’s various light paths are analogous 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, however, takes a different route, and the distance for each route is not the same. Some trains travel over hills. Others go through valleys, and still others chug around mountains. Because the trains travel over different track lengths across different topologies, they do not arrive at their destination at the same time. Similarly, the supernova images do not arrive at Earth at the same time because some of the light is delayed by traveling around bends created by the gravity of dense dark matter in the intervening galaxy cluster.
“Our model for the dark matter in the cluster gives us the prediction of when the next image will appear because it tells us how long each train track is, which correlates with time,” said Steve Rodney of Johns Hopkins University, leader of the FrontierSN team. “We already missed one that we think appeared about 20 years ago, and we found these four images after they had already appeared. The prediction of this future image is the one that is most exciting because we might be able to catch it. We hope to come back to this field with Hubble, and we’ll keep looking to see when that expected next image appears.”
Measuring the time delays between images offers clues to the type of warped-space terrain the supernova’s light had to cover and will help the astronomers fine-tune the models that map out the cluster’s mass. “We will measure the time delays, and we’ll go back to the models and say your prediction says the track would be this long and the hill would be this high,” Kelly said. “The lens modelers, such as Adi Zitrin (California Institute of Technology) from our team, will then be able to adjust their models to more accurately recreate the landscape of dark matter, which dictates the light travel time.”
While making a routine search of the GLASS team’s data, Kelly spotted the four images of the exploding star on Nov. 11, 2014, in the galaxy cluster MACS J1149.6+2223, located more than 5 billion light-years away. The FrontierSN and GLASS teams have been searching for such highly magnified explosions since 2013, and this object is their most spectacular discovery. The supernova appears about 20 times brighter than its natural brightness, due to the combined effects of two overlapping lenses. The dominant lens is due to the massive galaxy cluster, which focuses the supernova light along at least three separate paths. A secondary lensing effect occurs when one of those light paths happens to be precisely aligned with a single elliptical galaxy within the cluster. “The dark matter of that individual galaxy then bends and refocuses the light into four more paths,” Rodney explained, “generating the rare Einstein Cross pattern we are currently observing.”
The astronomers nicknamed the supernova Refsdal in honor of Norwegian astronomer Sjur Refsdal, who, in 1964, first proposed using time-delayed images from a lensed supernova to study the expansion of the universe. “Astronomers have been looking for one ever since,” said Treu. “The long wait is over!”
The Frontier Fields is a three-year program that teams Hubble with six massive galaxy clusters to probe not only what is inside the clusters but also what is beyond them through gravitational lensing. The GLASS survey is using Hubble’s spectroscopic capabilities to study remote galaxies through the cosmic telescopes of 10 massive galaxy clusters, including the six in the Frontier Fields.