Galactic Mysteries
Trove of far-flung galaxy clusters offers clues to dark matter and dark energy
A massive, young galaxy cluster seen in X-rays (blue), visible light (green), and infrared light (red). Image by X-Ray: NASA/CXC/Univ of Missouri/M. Brodwin et al.; optical: NASA/STSCI; infrared: JPL/Caltech
It’s a cosmic irony: the biggest things in the universe can also be the hardest to find.
Elizabeth Blanton, an associate professor of astronomy, started hunting for distant galaxy clusters more than 20 years ago. A single galaxy cluster—hundreds, even thousands, of galaxies held together by gravity—can be as massive as a quadrillion suns, yet faraway clusters are so faint that they are practically invisible to all but the biggest Earth-bound telescopes. Distant clusters hold pieces of the story of how the web-like structure of the universe first emerged and could help illuminate the true nature of dark energy and dark matter. Now, her team’s search is delivering its biggest return yet: a catalog of about 200 candidate clusters that, if confirmed, may include some of the most distant clusters ever found. The results were published in the July 20, 2017, edition of the Astrophysical Journal by a team that includes Rachel Paterno-Mahler (GRS’15), PhD candidate Emmet Golden-Marx (GRS’16,’19), Gagandeep Anand (GRS’17), Joshua Wing (GRS’07,’13), and colleagues at the University of Missouri-Kansas City and the Harvard-Smithsonian Center for Astrophysics.
Galaxy clusters can contain many trillions of stars—and that’s just what astronomers can see with ordinary telescopes. Hot gas between the galaxies glows with X-rays, and astronomers estimate that more than 85 percent of every cluster’s mass is hidden in the form of dark matter.
Together, dark matter and dark energy make up some 95 percent of our universe, scientists suspect, but astrophysicists know of dark matter’s and dark energy’s existence only indirectly, by their influence on the stars and galaxies that light up the sky. Dark matter is the invisible stuff that permeates galaxies and the spaces between them; dark energy is thought to drive the accelerating expansion of the universe.
The new cache of distant galaxy cluster candidates may help researchers pin down the properties of dark matter and dark energy, says Paterno-Mahler, a postdoctoral researcher at the University of Michigan and first author on the new paper, one of a series forthcoming from Blanton’s team. “Galaxy clusters are really good test-beds for learning about the cosmological parameters of our universe, like how much dark energy there is and how much dark matter there is.”
By comparing faraway clusters with their local counterparts, researchers can also assemble a timetable of how galaxy clusters formed and grew. That’s because light from the most distant clusters had to travel billions of years before reaching Earth, so astronomers see those clusters as they were long ago. And because galaxy clusters give astronomers access to a large sample of galaxies that are similar in age, they also provide a laboratory in which to study how individual galaxies have changed over time.
But the farther a galaxy cluster is from Earth, the fainter it appears. Traditional optical telescopes must stare at a single spot in the sky for a long time to collect enough light to reveal a distant cluster, and surveying the whole sky this way is time-prohibitive. So, to create the new catalog, Blanton and her team scoured archived data for clues to where clusters might be, and then followed up with targeted telescope observations. The search was supported by grants from the National Science Foundation and NASA.
Their trail of clues starts with the fact that almost every large galaxy has a supermassive black hole at its center. These black holes are notoriously messy eaters, and when they are feasting, some of the dust and gas plunging inward gets splashed out in enormous, spiraling jets. These jets can stretch the width of the galaxy and beyond, and they produce a radio-wave roar that astronomers can pick up with radio telescopes on Earth. If the galaxy also happens to be zooming through hot gas (or if the gas is zooming past the galaxy), the jets flex into a characteristic “C” shape—“like your hair blowing in the wind,” says Blanton. This “C” shape is the first clue to a possible cluster.
Blanton’s team pored over existing sky surveys and found almost 2,000 of these peculiar objects. Then Wing, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, compared (as part of his PhD work with Blanton) those suspected clusters to visual-light images from the archives of the Sloan Digital Sky Survey, which uses an optical telescope at Apache Point Observatory in New Mexico. The most exciting candidates, says Blanton, are those for which the Sloan pictures look dark, hinting that the radio signal could be coming from a cluster so distant that the Sloan Survey telescope can’t see it at all.
With the possibilities narrowed down further, they then used NASA’s infrared Spitzer Space Telescope to zero in on about 650 possible clusters, one by one. With a computer’s help, they counted up the number of galaxies in each Spitzer frame and compared it to the typical number of galaxies in a comparable area of the sky. An unusually high number of galaxies—called an “overdensity”—suggests a galaxy cluster.
An overdensity is not definitive proof of a galaxy cluster, though. “You’re seeing a 2-D image of a 3-D distribution of objects,” explains Blanton. “Some of them could be way in the foreground, or way in the background.” These “projection effects” can create the illusion of a cluster where none really exists. The group’s next step, under way now, is to measure the distance to each galaxy in the apparent cluster to confirm that the grouping is real, not an optical illusion.
Golden-Marx is already determining distances to some of the galaxies using the 4.3-meter Discovery Channel Telescope in Arizona, where BU is a partner institution, and Blanton hopes to secure time on the Hubble Space Telescope and one of the twin 10-meter Keck telescopes in Hawaii to get even more precise measurements. Once the distances are confirmed, the team will be able to properly order the clusters by age and also confirm whether their catalog really includes the most distant clusters yet found.