Ever since its launch in 2013 the European Space Agency’s Gaia spacecraft has been billed as a mission destined to transform our understanding of our galaxy. Tasked with tracking the position, motion, brightness and color of more than a billion stars, Gaia’s primary purpose is to create a dynamic three-dimensional map of the Milky Way that will be a standard for galactic cartography for generations to come.
But with the mission’s latest catalogue of data, released last April, astronomers are also using Gaia’s precision mapping to infer more about the nature of the mysterious dark matter that permeates the cosmos. “Gaia was built to measure the structure of our galaxy,” says Kathryn Johnston, an astronomer at Columbia University who has worked with the new Gaia data. “But the first key and most exciting results are not about the global structure at all—they are about the abnormalities on top of the expected structure.”
Like most large galaxies, the Milky Way grew via cannibalism, eating unlucky smaller neighbors—some of which are still being “digested,” slowly merging into our galaxy’s prevailing architecture. These scraps from the Milky Way’s gluttony all appear in Gaia’s latest data release as gravitationally perturbed structures such as stellar streams, debris fields and ultradiffuse dwarf galaxies—and each is associated with its own tagalong contingent of dark matter that Gaia’s precise measurement is uniquely poised to unveil.
In some respects these studies are scarcely different from those of the 1930s that first revealed dark matter’s existence. Astronomers observed stars whirling around the outskirts of other galaxies at speeds much faster than expected, as if influenced not only by the collective gravitational tug of a galaxy’s visible contents—its stars, gas and dust—but also by a surrounding “halo” of invisible material. Decades of further study have all but confirmed galaxies are nested within such dark matter halos, which extend out 10 times farther than a galaxy’s most distant stars. Dark matter’s distribution within a galaxy, especially towards its center, is still largely unknown, however.
Unraveling the finer details of dark matter’s intragalactic prevalence could lift the curtain on how exactly the Milky Way itself first came to be, which could in turn test researchers’ best models of galaxy formation and evolution. Galaxies, such models say, formed when gravity amplified slight fluctuations in the density of primordial gas that suffused the early universe, causing these regions to become denser and denser until stars, galaxies and clusters of galaxies emerged. Crucially, this scenario also demands less dense areas become even less dense, leaving many unable to form stars. Yet these starless regions should still contain approximately a hundred million times the sun’s weight in dark matter.
CLUMPS AND STREAMS
Such clumps have never been directly observed but their existence scattered around a galaxy’s exterior is a crucial cosmological prediction. Discovering one would go a long way toward showing our understanding of the early universe is correct—which is exactly what Gaia’s observations of stars at the inner edge of the Milky Way’s dark-matter halo might offer. The halo is populated with globular clusters—small, densely packed groups of old stars that orbit our galaxy.
Occasionally, one cluster may pass too close to another cluster and send it falling toward the galactic center. As it falls, gravity pulls more on its leading side than its trailing side, stretching the cluster out into a “tidal stream” of stars thousands of light-years long. On its long plunge from the halo such a stream could conceivably encounter and interact with one or more clumps of dark matter. “Streams are one of the most promising directions for testing the existence of small-scale dark matter subhalos (clumps),” says Adrian Price-Whelan, an astronomer at Princeton University. Along with collaborator Ana Bonaca, an astronomer at Harvard University, Price-Whelan sifted through Gaia’s fresh data in search of just such an interaction, focusing on one of the Milky Way’s longest, oldest tidal streams, a stretched-out band of stars known as GD-1 falling from the halo from high above the galactic disk.
Gaia enabled Price-Whelan and Bonaca to confidently separate the stars belonging to GD-1 from the foreground stars in the Milky Way, unambiguously revealing the existence of multiple gaps in the stream as well as a new feature never before seen: a spurlike offshoot of stars somehow pushed off the main track. A single gap is easily attributable to an interaction with another globular cluster but multiple gaps together with a spur suggest the stream has interacted with a very dense and compact object, something in the range of one million to 100 million solar masses—exactly the right range for a putative clump of dark matter soaring through the halo. A team led by Bonaca then retraced GD-1’s plunge from the halo, scouring the seemingly empty space for any other objects that could have caused the gaps and the spur. That search came up empty, leaving dark matter the most likely answer—for now. “The authors have convincingly shown that this spur can be attributed to an unseen clump of dark matter barreling past the stream at some speed, distorting its path,” Johnston says. “These dark matter clumps have been theorized to exist on such mass scales, but this is the most convincing evidence yet that we have detected their presence.”
DIFFUSE DWARF GALAXIES
Whereas the GD-1 stream is situated directly above the disk of the Milky Way, making it very easy to see, researchers using Gaia found another odd structure in one of the hardest areas to study: the far side of our galaxy, on the opposite side of the Milky Way’s star-clogged disk from the perspective of our solar system. Working with his colleagues, Gabriel Torrealba, an astronomer at the Institute of Astronomy and Astrophysics in Taiwan, used Gaia data to peer through the disk in search of previously unknown satellite galaxies on the far side of the Milky Way. Gaia’s exquisite measurements enabled the team to identify and screen out intervening stellar populations in the disk, revealing what was beyond them. What they found was Antlia 2 (Ant 2)—the most diffuse companion to the Milky Way ever detected.
Dubbed a “ghost galaxy,” Ant 2 is about three times fainter than other satellite galaxies of its size but larger than most other faint galaxies known to date. Ant 2’s stars are so scattered astronomers struggle to account for its existence. “Ant 2 is rather unexpected,” Johnston says. “I am surprised that such a diffuse galaxy could survive hanging out around the Milky Way.” In trying to understand how Ant 2 came to be, Torrealba and his team turned their attention to its potential inventory of dark matter.
Satellite galaxies are thought to be embedded in their own dark matter halos, which should be gradually stripped away from the outside in as the satellite galaxy falls into a larger one such as the Milky Way. Torrealba and colleagues suspect Ant 2 is so very diffuse because it is extremely impoverished of dark matter; if true, this along with Ant 2’s location on the periphery of our galaxy would suggest this ghostly galaxy has likely orbited the Milky Way several times already and is in the final stages of absorption after already having the majority of its dark matter siphoned away during earlier passes. Indeed, Torrealba guesses it will completely disintegrate on its next circuit.
Ant 2’s apparent lack of dark matter could prove to be a bonus for experimentalists hoping to detect dark matter via methods other than its gravitational interactions. For instance, the leading theory for dark matter’s physical identity is that it is composed of hypothetical weakly interacting massive particles (WIMPs). If and when WIMPs collide with each other somewhere in the cosmos, they should annihilate in a burst of energy that emits gamma rays. Torrealba and colleagues contend that, as stripped of dark matter as Ant 2 might be, it is more impoverished still of stars and gas that could otherwise generate gamma rays through various, more conventional astrophysical processes—so if any gamma rays are observed emanating from this ghost galaxy, they could be attributable to dark matter. Extending this to other recently discovered ultradiffuse galaxies we might find they collectively constitute a new, unprecedented probe for seeking WIMPs and other varieties of hypothesized dark matter particles.
Fortunately, we might not have to gaze all the way across the galaxy to put our dark matter hypotheses to the test. Thanks to an even older galaxy merger in the Milky Way recently discovered by Gaia, we can look for signs of it in our own “neighborhood,” the part of the galactic disk that holds our solar system.
Just over a year ago Vasily Belokurov, an astronomer at the University of Cambridge and a co-author with Torrealba on Ant 2, discovered signs in earlier Gaia data that the Milky Way had long ago experienced a major merger event with a galaxy of its own size. Because it was found using Gaia data, he nicknamed this hefty galactic interloper the Gaia Sausage. Of the billions of stars it brought crashing into Milky Way, some are now clustered in our solar system’s galactic backyard, perhaps with their associated dark matter lurking close by. But whereas we can identify the debris field of visible matter contributed by the Gaia Sausage merger, pinning down the dark matter that came with it has proved to be a challenging task.
Lina Necib, an astronomer at the California Institute of Technology, is hoping to make it easier. Using computer simulations, she and her team have modeled the coevolution of visible and dark matter from two merging galaxies much like the Gaia Sausage and Milky Way, looking to see if the two divergent types of matter end up with similar distributions. Their results suggest—although visible matter lags behind dark matter in the ensuing galaxy-spanning redistribution of material—by a merger’s conclusion both types of matter were whizzing around at similar, correlated speeds. For Necib and her team, this correlation means it should be possible to use the stars, gas and dust associated with the Gaia Sausage to trace the distribution of its corresponding dark matter. “Being able to track the dark matter component from recent mergers using their stellar counterparts is huge!” Necib says. And a reservoir of dark matter so close by could be even more useful for testing direct detection experiments using gamma rays, just as with Ant 2.
Thanks to Gaia, astronomers are amassing an array of techniques to probe our galaxy, dark matter and all, creating a new toolset with which to confirm or rule out some of the many theories describing this most elusive substance. GD-1 is one of about thirty streams, Ant 2 is one of roughly sixty Milky Way satellites, and the solar neighborhood is just one of the debris fields associated with the Gaia Sausage—all of which could be just the tip of the iceberg for their respective categories, leading to new studies on not just individual structures but their populations as a whole. Gaia’s mission is not yet complete—it is set to gather data through 2020, and appears healthy enough to continue operations into the middle of the next decade. But its legacy—revealing dark matter’s role in heretofore hidden chapters of the Milky Way’s history—is already crystal clear.