Dr. Michael McDonald of MIT’s Kavli Institute for Astrophysics and Space Research and co-authors have uncovered a new massive cluster of galaxies hiding in plain sight.
This image shows the newly-discovered galaxy cluster with the central giant elliptical galaxy, which harbors a supermassive black hole. Image credit: Taweewat Somboonpanyakul.
In 2012, Dr. McDonald and colleagues discovered the Phoenix cluster, one of the most massive and luminous galaxy clusters in the Universe.
The mystery to the astronomers was why this cluster, which was so intensely bright and in a region of the sky that is easily observable, hadn’t been found before.
“We started asking ourselves why we had not found it earlier, because it’s very extreme in its properties and very bright,” Dr. McDonald said.
“It’s because we had preconceived notions of what a cluster should look like. And this didn’t conform to that, so we missed it.”
“For the most part, astronomers have assumed that galaxy clusters look ‘fluffy,’ giving off a very diffuse signal in the X-ray band, unlike brighter, point-like sources, which have been interpreted as extremely active quasars or black holes.”
“The images are either all points, or fluffs, and the fluffs are these giant million-light-year balls of hot gas that we call clusters, and the points are black holes that are accreting gas and glowing as this gas spirals in. This idea that you could have a rapidly accreting black hole at the center of a cluster — we didn’t think that was something that happened in nature.”
“But the Phoenix discovery proved that galaxy clusters could indeed host immensely active black holes, prompting us to wonder: could there be other nearby galaxy clusters that were simply misidentified?”
To answer that question, Dr. McDonald and co-authors set up the Clusters Hiding in Plain Sight (CHiPS) survey, which is designed to reevaluate X-ray images taken in the past.
“We start from archival data of point sources, or objects that were super bright in the sky. We are looking for point sources inside fluffy things,” said Taweewat Somboonpanyakul, a graduate student at MIT.
For every point source that was previously identified, the astronomers noted their coordinates and then studied them more directly using the Magellan Telescope in Chile.
If they observed a higher-than-expected number of galaxies surrounding the point source (a sign that the gas may stem from a cluster of galaxies), they looked at the source again, using NASA’s Chandra X-Ray Observatory, to identify an extended, diffuse source around the main point source.
The CHiPS team’s first discovery is a massive galaxy cluster approximately 2.4 billion light-years away.
The cluster is made up of hundreds of individual galaxies and surrounds a powerful quasar called PKS1353-341.
The central quasar is intensely bright — so bright that for decades astronomers observing it in the night sky have assumed that the quasar was quite alone in its corner of the Universe, shining out as a solitary light source from the center of a single galaxy.
In their analysis, the researchers estimate that there are hundreds of individual galaxies in the cluster, which, all told, is about as massive as 690 trillion suns. Our Milky Way Galaxy, for comparison, weighs in at around 400 billion solar masses.
They also calculate that the PKS1353-341 quasar is 46 billion times brighter than the Sun.
Its extreme luminosity is likely the result of a temporary feeding frenzy: as an immense disk of material swirls around the quasar, big chunks of matter from the disk are falling in and feeding it, causing the black hole to radiate huge amounts of energy out as light.
“The brightness of the black hole might be related to how much it’s eating,” Dr. McDonald said.
“This is thousands of times brighter than a typical black hole at the center of a cluster, so it’s very extreme in its feeding. We have no idea how long this has been going on or will continue to go on. Finding more of these things will help us understand, is this an important process, or just a weird thing that there’s only one of in the Universe.”
The discovery is reported in a paper published in the August 16, 2018 issue of the Astrophysical Journal (arXiv.org preprint).
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Taweewat Somboonpanyakul et al. 2018. The Clusters Hiding in Plain Sight (CHiPS) Survey: A First Discovery of a Massive Nearby Cluster around PKS 1353−341. ApJ 863, 122; doi: 10.3847/1538-4357/aace55
