Through lenses of dark matter
Posted on: January 8, 2016; Updated on: January 8, 2016
By Steven Powell, spowell2@mailbox.sc.edu, 803-777-1923
Steve Rodney is using modern astronomy to bring into focus — or foci, to be precise — something that literally happened a long time ago, in a galaxy far, far away. He’s one of the leaders of a team using the Hubble Space Telescope to study multiple images of a beyond-ancient supernova that just recently became visible in our neck of the universe.
A supernova is a spectacular event, the explosion of a massive star at the end of its life cycle that radiates huge amounts of energy in a relatively short period of time. This one was rendered particularly special because of the arduous paths that its emanating rays of light have had to travel to make it to our solar system.
For more than two years now, Rodney has led an effort, called FrontierSN, to detect light from supernovae emanating from distant galaxies with what one might call a useful obstacle in the way.
“What we’re looking for are supernovae behind strong-lensing galaxy clusters,” Rodney says. “If you have a collection of galaxies with a lot of dark matter, it causes a general relativistic effect, a bending of light due to the mass of the dark matter and the galaxies. That can make objects behind the cluster very bright, and it can magnify them.”
The team found one such supernova in November 2014, while Rodney was still a post-doc at Johns Hopkins (he joined the Carolina faculty last fall). And it offered astronomers a view they had never seen before.
“We saw this beautiful array of four supernova images, in the shape of what’s often called an Einstein cross,” Rodney says. “That’s what happened because it was lined up very precisely behind the gravitational lens. We saw four images, like the four points on a compass, each being an image of the same supernova.”
The team has been following the waxing and waning of the images with periodic observations from Hubble, finding that maximum intensity occurred in April. There were small differences in time of maximum brightness among the four images, though, because the focused rays of light took slightly different paths in traversing the several billion light-years between where the supernova occurred and our solar system.
The team recently published in arXiv an analysis of how the light intensity varied over time for the four images, finding very good agreement between gravitational modeling of data and what was observed.
They also had to weather a bit of uncertainty at the end of the year. The models had also predicted that a fifth image of the supernova would appear nearby, delayed by an even-longer route than the ones taken by the light rays making four images in the Einstein cross.
“Colleagues who do gravitational lens modeling had been looking at the cluster, and they came to understand that the galaxy that the supernova appeared in had multiple images of its own,” Rodney says. “So there is a small gravitational lens due to one particular galaxy in the cluster, and then there’s a larger lens due to all the galaxies and the dark matter in the cluster.”
The models predicted that the new image of the supernova would have a maximum intensity in early 2016, so they expected it would first be visible in 2015. Because the sun was in the way between July and September and owing to time sharing on the Hubble, the first two observational windows to look at the region in question were in October and November. No new supernova image appeared either time.
But during the team’s Hubble observing window in December, the fifth supernova image appeared, exactly where it was supposed to. It arrived at the tail end of the models’ proposed time frame and was the first time a supernova observation has ever been predicted in advance.
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