Tiny Dwarf Galaxy Located in its First, High-Energy Stages of Star Formation

An enormous cluster of galaxies has been used as an X-ray magnifying glass by astronomers from MIT and elsewhere, to look back in time almost 9.4 billion years ago. During the study, they located a small dwarf galaxy in its initial, high-energy stages of star formation.

Although researchers have used galaxy clusters to magnify objects at optical wavelengths even earlier, this is the first time they have taken advantage of these enormous gravitational giants to zoom in on distant, extreme, X-ray-emitting phenomena.

The phenomena observed by the researchers appear as a blue speck of a young galaxy, with a size of about 1/10,000 of that of the Milky Way, in the process of producing its first stars. These stars are cosmically short-lived, supermassive objects that release high-energy X-rays, which was detected by the researchers in the form of a bright blue arc.

It’s this little blue smudge, meaning it’s a very small galaxy that contains a lot of super-hot, very massive young stars that formed recently. This galaxy is similar to the very first galaxies that formed in the universe ... the kind of which no one has ever seen in X-ray in the distant universe before.

Matthew Bayliss, Research Scientist, Kavli Institute for Astrophysics and Space Research, MIT

According to Bayliss, the discovery of this single, faraway galaxy is an evidence for the fact that it is possible for researchers to use galaxy clusters as natural X-ray magnifiers for the detection of extreme, highly energetic phenomena that occurred in the early history of the universe.

“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,” stated Bayliss, who will be moving on to the University of Cincinnati as an assistant professor of physics.

He and his colleagues, including Michael McDonald, assistant professor of physics at MIT, have reported the study outcomes in the Nature Astronomy journal on October 14^th, 2019.

A Candle in the Light

Galaxy clusters, which are formed of thousands of galaxies, are the most enormous objects in the universe. The galaxies in these clusters are all held together by gravity as one massive, powerful force.

Galaxy clusters are so enormous, with such a strong gravitational pull, that they have the ability to distort the space-time fabric, thereby bending the universe and any surrounding light, quite similar to the way an elephant stretches and warps a trapeze net.

Using a method called gravitational lensing, researchers have employed galaxy clusters as cosmic magnifying glasses. The underlying concept is that if researchers can estimate a galaxy cluster’s mass, they can approximate its gravitational effects on any surrounding light, together with the angle at which a cluster might deflect that light.

For example, consider an observer facing a galaxy cluster and attempting to detect an object, like a single galaxy, behind that cluster. The light discharged from that object would move straight toward the cluster, subsequently bending around the cluster.

It would continue its movement toward the observer, albeit at somewhat different angles, which appears to the observer as mirrored images of the same object. Eventually, these mirrored images can be merged as a single, “magnified” image.

Although galaxy clusters have been used by researchers to magnify objects at optical wavelengths, never have they used them in the X-ray band of the electromagnetic spectrum. This is specifically because galaxy clusters themselves discharge a huge amount of X-rays. Researchers considered that any X-rays emitted from a background source would not be possible to distinguish from the own glare of the cluster.

If you’re trying to see an X-ray source behind a cluster, it’s like trying to see a candle next to a really bright light. So we knew this was a challenging measurement to make.

Matthew Bayliss, Research Scientist, Kavli Institute for Astrophysics and Space Research, MIT

X-Ray Subtraction

The scientists were curious whether it would be possible to subtract that bright light and observe the candle behind it? Put differently, would it be possible to eliminate the X-ray emissions from the galaxy cluster to see the much weaker X-rays emitted from an object, behind and magnified by the cluster?

The researchers put this concept to test with observations by NASA’s Chandra X-ray Observatory, one of the most powerful X-ray space telescopes in the world. Specifically, they took a look at Chandra’s measurements of the Phoenix cluster, a faraway galaxy cluster 5.7 billion light-years from Earth, predicted to be about a quadrillion times as huge as the sun, possessing gravitational effects that should make it a robust, natural magnifying lens.

“The idea is to take whatever your best X-ray telescope is—in this case, Chandra—and use a natural lens to magnify and effectively make Chandra bigger, so you can see more distant things,” stated Bayliss.

He and his co-authors studied the observations of the Phoenix cluster, captured continuously by Chandra for more than a month. In addition, they analyzed images of the cluster captured by two optical and infrared telescopes—the Hubble Space Telescope and the Magellan telescope in Chile.

Using all these different observations, the researchers created a model to characterize the optical effects of the cluster, which enabled the researchers to accurately measure the X-ray emissions from the cluster itself, and subtract it from the data.

They obtained two identical patterns of X-ray emissions surrounding the cluster, which according to them were gravitationally bent, or “lensed,” by the cluster. Upon tracing the emissions backward in time, it was discovered that all the emissions arose from a single, faraway source: a small dwarf galaxy from 9.4 billion years ago, when the age of the universe itself was approximately around 4.4 billion years—nearly one-third of its present age.

“Previously, Chandra had seen only a handful of things at this distance,” stated Bayliss. “In less than 10 percent of the time, we discovered this object, similarly far away. And gravitational lensing is what let us do it.”

The combination of the Phoenix cluster’s natural lensing power and the Chandra X-ray Observatory allowed the researchers to observe the tiny galaxy hidden behind the cluster, magnified nearly 60 times. With this resolution, they could zoom in to distinguish between two different clumps inside the galaxy, where one generated much more X-rays compared to the other.

Since X-rays are essentially generated as part of extreme, short-lived phenomena, the scientists believe that the first X-ray-rich clump points to a part of the dwarf galaxy that has given rise to supermassive stars very recently, whereas the quieter region is an older one comprising of more mature stars.

We’re catching this galaxy at a very useful stage, where it’s got these really young stars. Every galaxy had to start out in this phase, but we don’t see a lot of these kinds of galaxies in our own neighborhood. Now we can go back in time, look in the distant universe, find galaxies in this early phase of their life, and start to study how star formation is different there.

Matthew Bayliss, Research Scientist, Kavli Institute for Astrophysics and Space Research, MIT

This study was partially funded by NASA and the Space Telescope Science Institute.