Seeing Quadruple, Seeing the Universe More Clearly

An exploding star has astronomers seeing quadruple—and they couldn’t be happier. Today in the AAAS journal Science, an international team of researchers led by Ariel Goobar at Stockholm University presents unique images of a special type of stellar explosion, called a Type 1a supernova, that will offer important new insights into gravity, dark matter, and the acceleration of the universe.

These images are so fascinating and scientifically valuable in part because they capture gravity in action. Before reaching the Earth, light from this supernova was warped by the gravity of a galaxy in its path. Through a process called gravitational lensing, light from the supernova was magnified and multiplied into four distinct images appearing around the “lens” galaxy.

The light from the supernova iPTF16geu and its host galaxy is warped and amplified by the curvature of space, owing to the mass of a foreground “lens” galaxy. In the case of the point-like supernova, the light is split into four images, which have been resolved with the Hubble Space Telescope.
Image Credit: ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson.

Another reason these images are so noteworthy is that they represent the first time astronomers have captured a gravitationally lensed Type 1a supernova. Caused by an exploding white dwarf star in a binary star system, Type 1a supernovae have a special place in astronomy. They are often called “standard candles” in reference to the fact that they always produce the same amount of light. This means that when astronomers detect a Type 1a supernova, they can figure out how far it is from us (and therefore how far its home galaxy is from us) by comparing its apparent brightness to its actual brightness. This unique feature led to the discovery of our universe’s accelerating expansion and the inference of dark matter.

Astronomers have been after a gravitationally lensed Type 1a supernova for a while, but the search has been difficult. Catching one on camera requires a combination of luck and quick action. It requires that the Earth, galaxy, and supernova be in precise alignment and that the supernova be identified and carefully observed before it fades from view.

What’s the big deal? Consider that astronomers now have four images of a Type 1a supernovae. Using existing techniques, they can find the distance to each supernova based on their apparent brightnesses. However, since all four images are of the same supernova, differences in distance are due entirely to this gravitational effect. From these differences, astronomers can calculate the expansion rate of the universe, described by the Hubble constant, without using any theoretical assumptions about the evolution of the universe.

This is an image of the gravitationally lensed iPTF16geu Type Ia supernova taken in near-infrared with the W.M. Keck Observatory. The lensing galaxy visible in the center has distorted and bent the light from iPTF16geu, which is behind it, to produce multiple images of the same supernova (seen around the central galaxy). The position, size and brightness of these images help astronomers infer the properties of the lensing galaxy.
Image Credit: W. M. Keck Observatory.

The imaged supernova, affectionally referred to as iPTF16geu, was first spotted last fall as part of the intermediate Palomar Transient Factory (iPTF) search led by Cal-tech. Instruments at Palomar Observatory scan the sky in this search, looking for objects that appear and disappear rapidly, such as supernovae. It was in this data that the team noticed a supernova that had the signature of a Type 1a, but it was 50 times too bright!

Realizing that this could be a gravitationally lensed supernova, the team worked quickly to secure time on instruments that could resolve the object more clearly. Follow-up observations in near-infrared light at Keck observatory and in optical light from the Hubble Space Telescope revealed the beautiful quadruple images that signal gravitational lensing.

We don’t yet know what the ongoing analysis of these images will tell us about the universe, but those of us at Physics Buzz are eagerly waiting to find out. We’ll let you know when we do!

Kendra Redmond

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