Hubble Spots a Lensed Supernova

For the first time, astronomers are watching as a supernova’s light bends around a massive galaxy on its way to Earth.

Einstein Cross by Hubble

The most famous example of an Einstein's Cross, this image shows the quadruple image of the quasar Q2237+0305.
NASA / ESA / STScI

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 gravity of the massive elliptical galaxy at the center of the cluster splits the supernova's light into four images. Additional images appear elsewhere, due to the cluster's gravitaty.NASA / ESA

The gravity of the massive elliptical galaxy at the center of the cluster splits the supernova's light into four images. Additional images appear elsewhere, due to the cluster's gravitaty.
NASA / ESA

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?

Supernova Refsdal

The light from supernova Refsdal (yellow) splits into four images as it passes through an elliptical galaxy in a massive cluster. The image S1 is in the upper right hand; S2, S3, and S4 follow clockwise.
NASA / ESA / S. Rodney & the FrontierSN team /T. Treu / P. Kelly & the GLASS team / J. Lotz & the Frontier Fields team / M. Postman & the CLASH team / Z. Levay

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

Galaxy cluster MACS J1149.5+223 is one of six deep fields Hubble is observing as part of its Frontier Fields program.NASA / ESA / S. Rodney & the FrontierSN team / T. Treu / P. Kelly & the GLASS team / J. Lotz & the Frontier Fields team / M. Postman & the CLASH team / Z. Levay

Galaxy cluster MACS J1149.5+223 is one of six deep fields Hubble is observing as part of its Frontier Fields program.
NASA / ESA / S. Rodney & the FrontierSN team / T. Treu / P. Kelly & the GLASS team / J. Lotz & the Frontier Fields team / M. Postman & the CLASH team / Z. Levay

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.

Reference:
Patrick Kelly et al. "Multiple Images of a Highly Magnified Supernova Formed by an Early-Type Cluster Galaxy Lens." Science, March 6, 2015.

6 thoughts on “Hubble Spots a Lensed Supernova

  1. Peter WilsonPeter Wilson

    In one NOVA episode, the producers poke fun at some of the ridiculously long 19th-century telescopes, made to try to overcome the color-dispersion of glass.

    They just weren’t thinking long enough. Using Galaxy cluster MACS J1149.5+223 as a lens is a bit awkward, but there’s zero color dispersion!

  2. Dieter Kreuer

    “That’s because as the universe expands, so does time — time runs slower for us than it did in the early universe.”

    I find this statement a bit misleading. First, it’s the time of the supernova that appears to run slower, not ours. And then, actually, time did run just as fast in the early universe as it does now, but the cosmological Doppler effect does not just stretch the wavelength of light through cosmic expansion, but the arrival time between any photons (or other signals, neutrinos, gravity waves, particles, whatever) from successive events reaching us from these vast distances. Cosmological Doppler as such is nothing but the apparent slowing of time flow caused by the expansion of space, stretching the space between any two signals sent out from the same origin at different times. Thus, it influences the duration of supernovae and gamma ray bursts just as as well as the wavelength of light.

  3. Ron-Skurat

    There’s no particular number of images that is “necessary,” is there? Wouldn’t a ring be observed in a perfectly symmetrical, idealized geometry?

    1. Mike-Jewell

      That’s exactly my question. Hard to imagine a symmetrical lensing mechanism that would produce four dots. And it seems like non-symmetrical and/or random density lenses would produce all manner of dots, arcs, etc.
      But the article seems to say that the “cross” or four dots is expected. I’d love a further explanation.
      Mike

  4. Howard RitterHoward Ritter

    Truly a beautiful demonstration, and image, of a rare phenomenon. It seems quite remarkable to me that all four images of the SN are visible simultaneously. The distance from the SN to us is estimated at 9.4B LY. Since the visible lifetime of the bright phase of the SN is on the order of one year, the path lengths of the light forming the four images must be equal to within ~ 10^-10, i.e. approximately 1 LY. In fact, since the images have been brightening over the four months of observation (six weeks at the SN itself), the SN must still be in the very short brightening phase of its light curve, suggesting a difference in path lengths of more like 0.1 LY at most, or one part in 10^11. Geometrically, the line connecting us to the SN seemingly must therefore be phenomenally close to the center of mass of the imaging galaxy. This does not seem to be the case from inspecting the discovery image. Is there something about the geometry of gravitational lensing that ensures that, for any location at which a source is imaged, the path lengths for all images of the source will be virtually equal? Or is this a remarkable coincidence? And if it is a coincidence, and most lensing cases would be expected to produce non-synchronous images perhaps years, decades, or centuries out of sync, wouldn’t this suggest that in some cases, a SN that appears to be in a distant galaxy is actually a single lensed image of a SN in a far more distant galaxy?

Comments are closed.