Using gravity to see stars further through space, time

STEVEN RODNEY / UNIVERSITY OF SOUTH CAROLINA

Distant stars come into focus when light bends around massive galaxies

Astronomers have imaged the furthest star yet discovered in the universe and documented an unusually quick brightening and dimming of a different star by using gravity as a magnifying lens. Researchers have given the stars nicknames: the extremely distant star is known as “Icarus” and the star with changing brightness is known as “Spock.”

Both papers, published in Nature Astronomy, use a process called gravitational lensing to see further back in time and space by using gravity of massive bodies to increase magnification of telescopes. Celestial objects which would be invisible can now be studied.

“We use the magnification to study the very first galaxies of the universe,” said Maruša Bradač, an associate professor in the UC Davis Department of Physics involved in both projects. “A team has selected six clusters to study very actively for that particular type of research. We use the galaxy clusters as lenses to study background galaxies.”

Massive galaxy clusters, each containing multiple galaxies and billions of stars, cause the light behind them to be bent and distorted by their immense gravity. Emitted light from stars, galaxies and other celestial structures further away are magnified and made visible to instruments such as the Hubble Space Telescope after bending around the huge clusters.

“They’re among the most massive structures in the universe,” said Austin Hoag, a Ph.D. candidate in the UC Davis Department of Physics and one of the team members of the Spock project. “All that mass makes them excellent tools for looking behind them and using them as natural magnifying glasses.”

Six clusters, called the Frontier Fields, were chosen to be observed for hundreds of hours by Hubble team members due to their exceptional mass and gravitational lensing capabilities.

“Around 2013, there was a new opportunity with Hubble,” said Steven Rodney, an assistant professor in the department of physics and astronomy at the University of South Carolina and one of the leaders of the Spock project. “The director at the time, Matt Mountain, had decided to invest a large amount of Hubble Telescope observing time in to what became the Frontier Fields project. The idea was that, using these gravitational lensing galaxy clusters, you could amplify Hubble’s capabilities. You could reach farther back in time, deeper into the universe, and see things that would normally be invisible, even to Hubble, by harnessing the power of the dark matter lenses that comprise the galaxy clusters.”

Icarus and Spock were spotted in images taken during the Frontier Fields observation studies, each being found in a respective galaxy cluster: MACS J1149 and MACS 0416.

“In the spring of 2016, I was looking through some of the images we got from Hubble and saw a new source in a special part of the field where the magnification from the cluster should be extreme,” said Patrick Kelly, an assistant professor in the School of Physics and Astronomy at the University of Minnesota and one of the leaders of the Icarus project. “It turned out that it had brightened, because a star orbiting around the cluster moved into the right spot and boosted the magnification by a factor of three, up to a total magnification of 2,000.”

For a brief time in May 2016, Icarus was bright enough for scientists to deeply investigate properties of the star. Icarus turned out to be the farthest star yet discovered.

“We measured its redshift, which tells you how much the universe has expanded since the light was emitted,” Kelly said. “We know the light was emitted 9.3 billion years ago, so the light has travelled 9.3 billion light years. That’s about three-fourths of the way back to the Big Bang. The next nearest star we can study individually is 100 times closer.”

After finding Icarus, Kelly was able to review past Hubble images taken in the same area to check if the star, a million times brighter than our sun, had appeared in previous observations. Hubble had been observing the cluster for a few years, allowing comparisons between exposures over time. Icarus is still available for study, but not at the same extreme magnification of 2,000 times.

“We can see [Icarus] all the time, if you look at it with Hubble,” Kelly said. “It’s always magnified by a factor of 600. That’s a difference with the other paper. With Spock, you can’t see anything there anymore.”

Spock is a star that briefly brightened and dimmed multiple times over a period of only a few weeks in 2014, enough time for researchers to notice interesting properties about the event.

“Spock was very unusual because it was very fast,” Rodney said. “It rose to its peak brightness in just a few days and faded away again, also in just a few days. It was all done in a matter of a couple of weeks. It was also unusual because it was fainter than a supernova but brighter than a nova.”

Mysteries remain about what exactly caused Spock to brighten so quickly. Brightening episodes due to stellar explosions or a planet impacting the star don’t seem to fit the timing pattern, or conflict with some of the information researchers received from Hubble studies.

“Spock is at what we call a redshift of one, which is about half the age of the universe,” Hoag said. “So that’s about seven billion years old.”

Having a telescope like Hubble in space has been a boon for stellar researchers. Terrestrial telescopes have trouble piercing the atmosphere with certain instruments and must also deal with light pollution from human civilizations.

“The Hubble Space Telescope images are what makes this possible,” Hoag said. “That’s because the Hubble Space Telescope has really good resolution, and it’s not affected by the atmosphere, which makes the resolution worse.”

The successor to the Hubble Space Telescope is deep in development. By 2020, astronomers and engineers hope to launch and deploy the James Webb Space Telescope, which will orbit the sun about 1.5 million kilometers from earth. The hope is that James Webb will be able to reach deeper into the universe, perhaps up to 13.4 billion years ago, when some of the first stars were forming. Hubble will continue to be used as a valuable scientific tool.

“We are pushing the envelope all the way to the very beginning of the universe,” Bradač said. “We study some of the first galaxies that formed in the universe, trying to figure out what they look like, what they’re made of. In particular, we’re trying to answer the big question: where did we come from?”

 

 

Written by: George Ugartemendia — science@theaggie.org