For the first time, astronomers are watching as a supernova’s light bends around a massive galaxy on its way to Earth.
Line up two objects just right, one in front of the other, and Einstein’s general theory of relativity serves up a treat: an Einstein cross, where the gravity from a foreground mass splits light from a background object into four separate images.
Until now, such crosses have always involved background quasars, whose brilliant beacons of light are powered by supermassive black holes. But in 1964, Sjur Refsdal (Hamburg Observatory, Germany) suggested a background supernova explosion could create a temporary cross, given the right line-up with a foreground galaxy.
Now, decades after Refsdal’s predictions, astronomers have finally struck gold. Patrick Kelly (University of California, Berkeley) and colleagues report in the March 6th Science Hubble Space Telescope observations of a supernova gravitationally lensed by a foreground elliptical galaxy in a massive galaxy cluster.
A Serendipitous Find
The supernova appears to lie in a spiral galaxy whose light has been traveling for some 9.4 billion years to reach us. About halfway here, this light passed by the massive elliptical galaxy, splitting into four separate magnified images.
It took two weeks of Hubble’s time, between November 3 and 20, 2014, to find the supernova’s images — half a century after Refsdal’s theoretical prediction. The authors named the supernova in Refsdal’s honor.
Yet in the two weeks of Hubble observations published in Science, only a week went by for Supernova Refsdal. That’s because as the universe expands, so does time — time runs faster for us than it did in the early universe. So, since supernovae don’t tend to vary dramatically during the course of a week, it’s unsurprising that only one of the four detected images (labeled S3) was found to vary in the limited exposure time.
Another image (labeled S4) is only barely detectable (it’s only magnified to be twice as bright by the lens, whereas the others are magnified 10 times).
Still, capturing the four images establishes two things. First, this is not a background quasar: comparison with archival observations attests to that. Second, since one of the images is brightening over time rather than fading, this can’t be a Type Ia supernova (the explosive death of a white dwarf), since it would already have started fading by now. So the light most likely comes from a collapsing star throwing off its outer layers.
Which Image Came First?
The images astronomers see depend on how mass is distributed in the lens, and that’s especially complicated here. The lensing galaxy is part of a larger galaxy cluster, so other, smaller galaxies, hot cluster gas, and surrounding dark matter all contribute their own gravitational influence on the light passing through.
Kelly’s team modeled the cluster, determining how it bends the supernova’s light. They think that the S1 image arrived first, then S2, and then either S3 or S4. Now, if Kelly’s team can measure the exact time-delays between the images’ arrival, they could accomplish Refsdal’s ultimate goal: an independent measure of the universe’s expansion.
The models also predict the arrival of a fifth image anytime from a year to a decade from now.
On the Frontier
But wait, there’s more: though not included in the Science paper, Kelly says Hubble has continued watching the field for an additional four months (and counting). That’s because the supernova and the galaxy cluster in front of it, MACS J1149.5+2223, lie in one of Hubble’s Frontier Fields, six deep fields centered on massive galaxy clusters. Hubble began observing that field in earnest only a week after the supernova’s discovery.
Again, because of the universe’s expansion, the four months of exposure time equate to only about a month and half in the supernova’s world. But, Kelly says, that time is enough to see that three of the four images have continued to brighten. (The fourth image remains too faint to accurately monitor changes in brightness.)
“This gradual increase in brightness suggests that the supernova is similar to the peculiar Type II supernova 1987A, the explosion of a massive blue supergiant star in the Large Magellanic Cloud,” Kelly says.
Kelly’s team continues to monitor the supernova via the Frontier Fields observations, hoping that Supernova Refsdal will stop brightening and start fading before the field goes behind the Sun from Hubble’s point of view. If this change occurs across all four images, the team can measure the time delay, and ultimately perhaps the universe’s expansion rate.
But Kochanek cautions that that will require understanding the lens really well — knowing how the density changes across the galaxy cluster to within 1% of the actual values. Because this lens is part of a massive galaxy cluster, “this is a really ugly place to try to get that precision.” To use the supernova as a cosmological tool, he adds, the team will have to measure the difference in arrival times between the four images to at least 5% precision.
Patrick Kelly et al. "Multiple Images of a Highly Magnified Supernova Formed by an Early-Type Cluster Galaxy Lens." Science, March 6, 2015.