What does it take to discover a galaxy? Fate, luck and star dust

Photo courtesy of James Josephides, Christina Williams and Ivo Labbé.

By Paige Colley
BU News Service

BOSTON — Galaxies are one of the largest structures in the universe. So it seems strange that a group of scientists could discover one by accident.

This year, a group of 10 scientists accidentally found a massive, old galaxy hiding in the night sky while conducting different research looking at why certain galaxies stop forming stars. And it might not be the only one.

“We found the object in the background that was basically invisible in all the other data sets that we had,” said Dr. Christina Williams, an astronomer from the University of Arizona, who was the lead author on the paper published in October in the Astrophysical Journal.

The galaxy was not visible in any other frequency wavelength, such as visible light, which humans can see with their own eyes, or infrared, a longer wavelength that can often be recorded as heat. It could only be seen in the long-wave radio frequencies the team had been using.

“It’s serendipitous, because you literally cannot find it in the other data,” said Dr. Kate Whitaker, one of the astronomers on the team who recently became an assistant professor at UMass Amherst.

Whitaker has two explanations for why that was.

“Because it’s so distant, but also because it’s so dusty. So these two things combined, make it so that it only emits a very long wavelength,” she said.

With distance and dust, the galaxy has managed to hide from view despite its massive size.

When the team started their original research, they drafted a proposal to use the Atacama Large Millimeter Array, a telescope in Chile made up of 66 different antennas. When combined, these antennae allow the telescope to see light with a wavelength of about a millimeter, which falls in the middle of the electromagnetic spectrum.

“If you want to look at dead galaxies, you really need the sensitivity,” Whitaker explained. Because the telescope was so large and precise, it could look at a tiny section of the sky in great detail. The trade-off was how much sky the scientists could look at.

The Atacama Large Millimeter Array is located in Chile, where the conditions for earth-based telescopes are perfect. Photo credit ESO/C. Malin

Their proposal was approved, and the team was granted two days of viewing in December 2018. From this data, the scientists were able to make what they describe as a chance discovery: a fuzzy little blob.

“First we were trying to figure out if it could be wrong because as you go to earlier times in the universe, it takes a long time for galaxies to get big,” Williams said. “When we measured how far away we thought the galaxy was, we were starting to worry that maybe it was too big to be that early in the universe.”

The original research that Williams and her team of six astronomers were doing focused on massive, old galaxies that no longer form stars. To do this, they look at radio waves to see how much gas old galaxies have, as gas is the key to galaxy formation.

The current theory on galaxy formation said that after the Big Bang, the universe was filled with gas. This gas began to condense, forming dense pockets of matter. When enough of this matter collected, it formed stars, and their gravitational pull attracted other stars. Once enough stars gathered, it became a galaxy. The more gas a galaxy had, the more potential it had to grow.

However, despite the abundance of gas, astronomers don’t understand why so many of these early universe galaxies stop forming stars, and Williams explained that is one big question astronomers want answered.

“A galaxy should keep getting bigger, but they don’t,” she said.

Early universe galaxies become massive early on, but then stop their star forming process. Their new discovery has given them a chance to take a closer look at these early universe galaxies. 

“We’re starting to sort of piece together a picture of how those galaxies are growing. And so it’s kind of like an important missing link that we found,” Williams said.

Looking at the blob, the team started to realize that the galaxy was bigger than they expected for such an old galaxy.

“What we know about cosmology would say that’s highly unlikely,” Williams said.

So the team decided to try and see if there were errors in the measurement.

The next step after this chance find was to recruit a few more researchers with specialties in measuring the position and brightness of the blob, which brought the total number of scientists on the project to 10.

“We did a lot of tests to try to convince ourselves,” Williams said. “And in the end, we convinced ourselves that it was probably pretty far away.”

While the discovery of this massive, ancient galaxy was by chance, it is by no means an anomaly.

“There’s been a couple of other papers recently, very recently, that are starting to find similar things,” Whitaker said. “We’re not the only ones who are finding this.”

Both scientists agree that improvements to telescopes will make these kinds of discoveries more frequent.

“If we had more data that goes as faint as the data set we made then we’d probably see more of them,” Williams said.

Luckily, the team won’t have to wait long as a new telescope will launch in less than two years.

The James Webb Space Telescope as it undergoes tests before being launched into space. Without the interference of the atmosphere on earth, space is an ideal place to put telescopes. Image credit NASA/Chris Gunn

“We need new technology, and that’s the James Webb Space Telescope,” Whitaker said, referencing a new space telescope planned to launch in 2021. The telescope, which some see as the successor to the Hubble Space Telescope, will have the ability to take photos of the oldest objects in the universe.

Williams intends to do further research on their discovery, and is currently working on a new proposal to use the Atacama Large Millimeter Array again. But for now, both astronomers plan on finishing what they had started before being sidetracked by this new galaxy. 

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