“We started asking ourselves why we had not found it earlier, because it’s very extreme in its properties and very bright,” says Michael McDonald, assistant professor of physics in MIT’s Kavli Institute for Astrophysics and Space Research about the discovery of a sprawling new galaxy cluster hiding in plain sight. “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.”
Led Astray by a Blazingly Bright Quasar
The cluster, which sits a mere 2.4 billion light years from Earth, is made up of hundreds of individual galaxies and surrounds an extremely active supermassive black hole, or quasar, that goes by the name PKS1353-341. The 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.
But as the MIT team reported in the Astrophysical Journal, the quasar’s light is so bright that it has obscured hundreds of galaxies clustered around it. An X-ray image (in blue) with a zoom in optical image (gold and brown) below shows the central galaxy of the hidden cluster, which harbors a supermassive black hole. (Taweewat Somboonpanyakul).
In their 2018 paper, MIT researchers estimated 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.
Galaxy clusters are sensitive probes into the very nature of the cosmos”
The team also calculates that the quasar at the center of the cluster 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.
Just a Blip?
“This might be a short-lived phase that clusters go through, where the central black hole has a quick meal, gets bright, and then fades away again,” says study author McDonald. “This could be a blip that we just happened to see,” McDonald notes.” In a million years, this might look like a diffuse fuzzball.”
McDonald and his colleagues believe the discovery of this hidden cluster shows there may be other similar galaxy clusters hiding behind extremely bright objects that astronomers have miscatalogued as single light sources. The researchers are now looking for more hidden galaxy clusters, which could be important clues to estimating how much matter there is in the universe and how fast the universe is expanding.
If we are missing even a small number of the total population we may infer a different universe than the one we live in”
The paper’s co-authors include lead author and MIT graduate student Taweewat Somboonpanyakul, Henry Lin of Princeton University, Brian Stalder of the Large Synoptic Survey Telescope, and Antony Stark of the Harvard-Smithsonian Center for Astrophysics.
In 2012, McDonald and others discovered the Phoenix cluster, one of the most massive and luminous galaxy clusters in the universe. The mystery to McDonald 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.
This Chandra image on the left shows the newly discovered Phoenix Cluster, located about 5.7 billion light years from Earth. This composite includes an X-ray image from NASA’s Chandra X-ray Observatory in purple, an optical image from the 4m Blanco telescope in red, green and blue, and an ultraviolet (UV) image from NASA’s Galaxy Evolution Explorer (GALEX) in blue. The Chandra data reveal hot gas in the cluster and the optical and UV images show galaxies in the cluster and in nearby parts of the sky.
For the most part, he says 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.
We didn’t think that was something that happened in nature”
“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,” McDonald says. “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 McDonald to wonder: Could there be other nearby galaxy clusters that were simply misidentified?
To answer that question, the researchers set up a survey named CHiPS, for Clusters Hiding in Plain Sight, 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,” Somboonpanyakul explains. “We are looking for point sources inside fluffy things.”
For every point source that was previously identified, the researchers noted their coordinates and then studied them more directly using the Magellan Telescope, a powerful optical telescope that sits in the mountains of 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), the researchers looked at the source again, using NASA’s space-based Chandra X-Ray Observatory, to identify an extended, diffuse source around the main point source.
“Some 90 percent of these sources turned out to not be clusters,” McDonald says. “But the fun thing is, the small number of things we are finding are sort of rule-breakers.”
The paper reports the first results of the CHiPS survey, which has so far confirmed one new galaxy cluster hosting an extremely active central black hole.
“The brightness of the black hole might be related to how much it’s eating,” McDonald says. “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 in the universe.”
The team plans to comb through more X-ray data in search of galaxy clusters that might have been missed the first time around.
“If the CHiPS survey can find enough of these, we will be able to pinpoint the specific rate of accretion onto the black hole where it switches from generating primarily radiation to generating mechanical energy, the two primary forms of energy output from black holes,” says Brian McNamara, professor of physics and astronomy at the University of Waterloo, who was not involved in the research. “This particular object is interesting because it bucks the trend. Either the central supermassive black hole’s mass is much lower than expected, or the structure of the accretion flow is abnormal. The oddballs are the ones that teach us the most.”
May Help Estimate How Fast the Universe is Expanding
In addition to shedding light on a black hole’s feeding, or accretion behavior, the detection of more galaxy clusters may help to estimate how fast the universe is expanding.
“Take for instance, the Titanic,” McDonald says. “If you know where the two biggest pieces landed, you could map them backward to see where the ship hit the iceberg. In the same way, if you know where all the galaxy clusters are in the universe, which are the biggest pieces in the universe, and how big they are, and you have some information about what the universe looked like in the beginning, which we know from the Big Bang, then you could map out how the universe expanded.”
The Last Word –Thousands of Previously-Unknown Clusters Discovered
In an email to The Daily Galaxy, physicist Antony Stark at the Harvard-Smithsonian Center for Astrophysics, who was a member of the discovery team, wrote: “Cosmic Microwave Background surveys at millimeter wavelengths by the South Pole Telescope and the Atacama Cosmology Telescope are discovering thousands of previously-unknown clusters of galaxies. The clusters are detected via the Sunyaev-Zel’dovich effect. For some regions of the sky (about 15% of the whole sky) we now have a complete census of all galaxy clusters above a certain mass at all redshifts. This result can be directly compared to what is expected from the current standard theory of cosmology, the Lambda Cold Dark Matter Big Bang, giving us accurate measures of the amount of matter in the Universe, its rate of expansion, and its eventual fate.”
“Since the original article was published, we have found at least one other cluster similar to the Phoenix cluster, MIT’s McDonald told The Daily Galaxy. “Clusters such as this are important,” he explains, “as they reveal biases in how we identify clusters — when they don’t look like what we expect, we may overlook them. As the largest bound objects in the Universe, galaxy clusters are sensitive probes into the very nature of the cosmos, but if we are missing even a small number of the total population we may infer a different universe than the one we live in.”
The image at the top of the page shows the Phoenix cluster, an enormous accumulation of about 1,000 galaxies, located 5.7 billion light years from Earth. At its center lies a massive galaxy, which appears to be spitting out stars at a rate of about 1,000 per year.
Hubble Image at the top of the page: was made by combining data from Chandra, Hubble and the VLA. X-rays from Chandra depict hot gas in purple and radio emission from the VLA features jets in red. Optical light data from Hubble show galaxies (in yellow), and filaments of cooler gas where stars are forming (in light blue).