“The universe is full of things that emit X-rays, and up until now, being able to pick out something like this has been completely out of reach of today’s telescopes,” said Gladders, a leading expert in the field of gravitational lensing. “This is a glimpse of what we’ll be able to see routinely with the next generation of telescopes.”
Bayliss, MSc’07, PhD’11, said the detection of this single distant galaxy is proof that scientists can use galaxy clusters as natural X-ray magnifiers, to pick out extreme, highly energetic phenomena in the universe’s early history.
“With this technique, we could, in the future, zoom in on a distant galaxy and age-date different parts of it—to say, this part has stars that formed 200 million years ago, versus another part that formed 50 million years ago, and pick them apart in a way you cannot otherwise do,” Bayliss said.
A candle in the light
Galaxy clusters are the most massive objects in the universe, composed of thousands of galaxies, all bound together by their mutual gravitational attraction.
The gravitational distortion of space-time that occurs near massive galaxy clusters, much like an elephant stretching a trapeze net, bends the paths of photons that pass by.
This phenomenon, known as gravitational lensing, creates an effect that scientists have used as cosmic “magnifying glasses.” It’s been done before with photons at optical wavelengths, but never with photons in the X-ray band of the electromagnetic spectrum, mainly because galaxy clusters themselves emit an enormous amount of X-rays. Scientists have thought that any X-rays coming from a background source would be impossible to discern from the cluster’s own glare.
The team tested this idea with observations taken by NASA’s Chandra X-ray Observatory, one of the world’s most powerful X-ray space telescopes (named for Nobel-winning UChicago astrophysicist Subrahmanyan Chandrasekhar).
They used the observatory to look at the Phoenix cluster, a distant galaxy cluster located 5.7 billion light-years from Earth discovered in 2011 by the South Pole Telescope, which is located at the Amundsen-Scott South Pole Station and run by a collaboration led by the University of Chicago.
The cluster has been estimated to be about a quadrillion times as massive as the sun, with gravitational effects that should make it a powerful natural magnifying lens.
They combined the Chandra observatory’s results with data on the cluster taken by two optical and infrared telescopes: the Hubble Space Telescope and the Magellan telescope in Chile. With all these various views, the team developed a model to characterize the cluster’s light-bending effects, and allowed them to precisely measure the X-ray emissions from the cluster itself, and subtract it from the data.
They were left with two similar patterns of X-ray emission around the cluster, which they determined were “lensed,” or gravitationally bent, by the cluster. When they traced the emission backward in time, they found that they all originated from a single, distant source: a tiny dwarf galaxy from 9.4 billion years ago, when the universe itself was roughly 4.4 billion years old—about a third of its current age.