Harvard researcher presents data on the makeup of the Milky Way

The galactic plane of the Milky Way as seen from planet Earth.
The galactic plane of the Milky Way as seen from planet Earth. (AFP/Getty Images)

There’s more to the Milky Way halo than meets the (trained) eye, according to a Harvard researcher.

Ana Bonaca, a postdoctoral fellow at the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, presented intriguing new findings about the makeup of the Milky Way last month at an academic conference.

Bonaca gave her presentation, dubbed “Dynamical evidence for a dark substructure in the Milky Way halo,” at the American Physical Society’s April meeting, according to the society’s website. Her findings were first reported by LiveScience and later picked up by Newsweek.

Here’s what she and her colleagues unearthed, according to a summary posted on the APS site.


“Stars escaping globular clusters form thin, long and kinematically-cold tidal streams. In pristine conditions, these streams have nearly uniform density, however, new Gaia observations of one such structure in the Milky Way halo have revealed a likely site of perturbation,” the summary says. “The on-sky morphology suggests a recent, close encounter with a massive and dense perturber. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate.”

That observation, the summary says, “opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo.”

Bonaca said Thursday in a telephone interview that she and her colleagues are in the process of obtaining more data.

“If this turns out to be a clump of dark matter, its mass will directly tell us something about the mass of the dark matter particle,” she said.

Physics, the online journal of the American Physical Society, also discussed the research in a recent posting.

“Bonaca and her colleagues looked at all of the known molecular gas clouds, globular star clusters, and dwarf galaxies that could have perturbed the stream but could not find anything having close to the correct combination of parameters,” the posting said. “A big clump of dark matter, on the other hand, seems to be the best option. A prediction from the standard cosmological model for the typical size of a dark matter clump comes closer to the correct parameters but is still a bit off.”


The mysterious dark matter is estimated to make up about 27 percent of the universe. Scientists are more certain about what it’s not than what it is, NASA says. It is a component of the universe that can be detected by its gravitational attraction but not seen, according to

Bonaca received her doctorate in astronomy in 2016 at Yale, according to her biography on the Harvard website.

She “studies 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,” the bio says. “In her work, she uses data from large photometric surveys, as well as high-resolution numerical simulations.”

The biography lists Bonaca’s research interests as “[t]idal disruption of Galactic satellites and buildup of its stellar halo, global mapping of dark matter in the Milky Way and searches for low-mass dark matter subhalos.”

Travis Andersen can be reached at Follow him on Twitter @TAGlobe.