Over the past several decades, astronomers have come to realise that the sky is filled with magnifying glasses that allow the study of very distant and faint objects barely visible with even the largest telescopes.
A University of California, Berkeley, astronomer has now found that one of these lenses — a massive galaxy within a cluster of galaxies that are gravitationally bending and magnifying light — has created four separate images of a distant supernova.
The so-called “Einstein cross” will allow a unique study of a distant supernova and the distribution of dark matter in the lensing galaxy and cluster.
“Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses,” said UC Berkeley postdoctoral scholar Patrick Kelly, who discovered the supernova while looking through infrared images taken November 10th, 2014, by the Hubble Space Telescope (HST). “That will be neat.”
Kelly is a member of the Grism Lens-Amplified Survey from Space (GLASS) team led by Tommaso Treu at UCLA, which has worked in collaboration with the FrontierSN team organised by Steve Rodney at Johns Hopkins University to search for distant supernovae.
“It’s a wonderful discovery,” said Alex Filippenko, a UC Berkeley professor of astronomy and a member of Kelly’s team. “We’ve been searching for a strongly lensed supernova for 50 years, and now we’ve found one. Besides being really cool, it should provide a lot of astrophysically important information.”
One bonus is that, given the peculiar nature of gravitational lensing, astronomers can tune in for a supernova replay within the next five years. This is because light can take various paths around and through a gravitational lens, arriving at Earth at different times. Computer modelling of this lensing cluster shows that the researchers missed opportunities to see the exploding star 50 years ago and again 10 years ago, but images of the explosion will likely repeat again in a few years.
“The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay,” noted Filippenko.
Kelly is first author of a paper reporting the supernova appearing this week in a special March 6th issue of Science magazine to mark the centenary of Albert Einstein’s general theory of relativity.
Kelly, Filippenko and their collaborators have dubbed the distant supernova SN Refsdal in honour of Sjur Refsdal, the late Norwegian astrophysicist and pioneer of gravitational lensing studies. It is located about 9.3 billion light-years away (redshift = 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light-years (redshift = 0.5) from Earth.
Einstein’s general theory of relativity predicts that dense concentrations of mass in the universe will bend light like a lens, magnifying objects behind the mass when seen from Earth. The first gravitational lens was discovered in 1979. Today, lensing provides a new window into the extremely faint universe shortly after its birth 13.8 billion years ago.
“These gravitational lenses are like a natural magnifying glass. It’s like having a much bigger telescope,” Kelly said. “We can get magnifications of up to 100 times by looking through these galaxy clusters.”
When light from a background object passes by a mass, such as an individual galaxy or a cluster of galaxies, the light is bent. When the path of the light is far from the mass, or if the mass is not especially large, “weak lensing” will occur, barely distorting the background object. When the background object is almost exactly behind the mass, however, “strong lensing” can smear extended objects (like galaxies) into an “Einstein ring” surrounding the lensing galaxy or cluster of galaxies. Strong lensing of small, point-like objects, on the other hand, often produces multiple images — an Einstein cross — arrayed around the lens.
“We have seen many distant quasars appear as Einstein crosses, but this is the first time a supernova has been observed in this way,” Filippenko said. “This short-lived object was discovered only because Pat Kelly very carefully examined the HST data and noticed a peculiar pattern. Luck comes to those who are prepared to receive it.”
The galaxy that is splitting the light from the supernova into an Einstein cross is part of a large cluster, called MACS J1149.6+2223, that has been known for more than 10 years.
In 2009, astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The new supernova is located in one of that galaxy’s spiral arms, which also appears in multiple images around the foreground lensing cluster. The supernova, however, is split into four images by a red elliptical galaxy within the cluster.
“We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system,” Kelly said.
Looking for Transients
After Kelly discovered the lensed supernova November 10th while looking for interesting and very distant supernova explosions, he and the team examined earlier HST images and saw it as early as November 3rd, though it was very faint. So far, the HST has taken several dozen images of it using the Wide Field Camera 3 Infrared camera as part of the Grism survey. Astronomers using the HST plan to get even more images and spectra as the telescope focuses for the next six months on that area of sky.
“By luck, we have been able to follow it very closely in all four images, getting data every two to three days,” he said.
Kelly hopes that measuring the time delays between the phases of the supernova in the four images will enable constraints on the foreground mass distribution and on the expansion and geometry of the universe. If the spectrum identifies it as a Type Ia supernova, which is known to have a relatively standard brightness, it may be possible to put even stronger limits on both the matter distribution and cosmological parameters.