Since the Gaia Spacecraft’s April 2018 data release, astronomers have unveiled a Milky Way teeming with astounding surprises, including hints of dark-matter clumps that might eventually provide a better grasp of the elusive material’s properties. These early, easy-to-spot findings, reports Adam Mann in Nature, have been “transformational.” Astronomer Vasily Belokurov at the University of Cambridge, UK, said they are merely a glimpse of what is to come: “How we see the Milky Way has clearly changed.”
Gaia’s precision will ultimately be 100 times greater than any previous effort. And thanks to its sensitivity, it can probe deeper into the Galaxy: some 99% of the more than 1 billion stars it observes have never had their distances accurately determined. Knowing where each star is, reports Nature, “and where it’s going allows researchers to tease out hidden Milky Way history”.
Gaia is forcing researchers to take a second look at some of the canonical assumptions that are used to simplify models, says astrophysicist Adrian Price-Whelan at Princeton University. “We knew those assumptions were wrong,” he adds. “Gaia has now shown us how wrong they were.”
Mapping the Milky Way’s stars could also shed light on dark matter: theorists suspect that our Galaxy sits inside an enormous, roughly spherical halo of dark matter that, much like ordinary matter, has clumped together into smaller structures thanks to gravity. Cosmological simulations suggest that thousands of large dark-matter clumps orbit the Galaxy, occasionally getting eaten by a mass of dark matter at the center, in a process akin to the Milky Way’s consumption of its small satellite dwarf galaxies.
The vast majority of the dark-matter substructures are thought to contain few or no stars make them hard to detect, but Gaia might have found a hint of one in GD-1, a long stream of stars discovered in 2006 that stretches across half of the northern sky. Gaia enabled Price-Whelan and astronomer Ana Bonaca at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to pick out true members of the group. The two study how the tidal field of the Milky Way galaxy disrupts globular clusters, and what the resulting debris can tell us about the underlying distribution of dark matter
Last November 2018, Price-Whelan and Bonaca and two other colleagues identified a distinct gap, that could be the scar of an encounter with a massive object some 500 million years ago. “As the dark object sped past the stream, it might have separated the train of stars by gravitationally tugging on some, reports Mann, allowing them to pull ahead of their companions.
“The most likely culprit seems to be a dense dark-matter clump, probably somewhere between 1 million and 100 million times the mass of the Sun,” says Bonaca, an estimate could have implications for physical models of dark matter. A dark-matter particle’s mass helps to dictate how fast it can move and, in turn, the size of clusters it is liable to form. The GD-1 perturber’s size is in an interesting range, says Bonaca, that could eliminate hypothesized dark-matter candidates that are relatively low in mass.
Bonaca and her team are now interested in using Gaia data to determine the orbit of the ancient dark-matter object. “If they can ascertain where it could be found today,” reports Nature, “they might be able to detect its gravitational effects on other material. Or perhaps they could train γ-ray telescopes on the spot to look for evidence of dark-matter particles annihilating one another or decaying, processes that could emit energetic photons. Either technique could offer a more-direct probe of the invisible substance’s physical properties.”
Princeton’s Price-Whelan hopes that systematic studies using the Gaia catalog and future observatories — such as the ground-based Large Synoptic Survey Telescope in Chile, which is expected to begin gathering data in the early 2020s — will reveal fainter stars and other stellar streams. If some of those streams also show effects from encounters with dark-matter clumps, he says, they could give astronomers a better idea of the abundance and size of such clusters, “which would help to pin down the properties of dark matter.”
Gaia’s data on stellar motions will help astronomers map the shape of the Galaxy’s dark side. Belokurov expects that new data from Gaia on local stellar orbits will be sufficient to trace out the overall mass and shape of the dark-matter halo in the next 2–4 years.
Image at the top of the page: When astrophysicists model the Milky Way’s gamma-ray sources to the best of their knowledge, they are left with an excess glow at the galactic center. Some researchers have argued that the signal might hint at hypothetical dark matter particles. However, it could also have other cosmic origins. (NASA; A. Mellinger/Central Michigan University; T. Linden/University of Chicago)