An enormous “something” more massive than a star, appears to have torn a hole in part of the Milky Way’s halo. The “dark substructure” was found in data from Gaia spacecraft observations—a mission producing the most detailed 3D map of our galaxy—with Harvard’s Ana Bonaca noticing a perturbation in a tidal stream. Bonaca is a leading authority on 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.(Each day, between Christmas and New Year’s Day, we’ll post one of The Daily Galaxy’s 2019 most viewed posts as ranked by Google Analytics. With our best wishes for The Holidays.)

Globular clusters are large congregations of stars, explains Bonaca, which gradually lose their members to form thin and long star streams. “In pristine conditions,” she observes, “these streams have a nearly uniform density, however, new observations of one such structure in the Milky Way halo have revealed a likely site of perturbation. The on-sky morphology suggests a recent, close encounter with a massive and dense perturber. This discovery opens up the exciting possibility that detailed observations of streams could measure the abundance of dark-matter substructure and thus shed light on the nature of dark matter.

The “perturber”, as Bonaca dubs it, is unknown –“It’s a dense bullet of something,” she told LiveScience. The hole is enormous, that whatever created it was much more massive than a star. “Something like a million times the mass of the sun. So there are just no stars of that mass. We can rule that out. And if it were a black hole, it would be a supermassive black hole of the kind we find at the center of our own galaxy.”

Telescope observations do not show any large luminous object moving away from the hole, which led Bonaca to suggest the perturbation could have been made by dark matter. “Observations permit a low-mass dark-matter subhalo as a plausible candidate,” Bonaca’s abstract says.

If it was a dense blob of dark matter that smashed through the tidal stream, it would be an exciting find for scientists, as it would provide them the opportunity of studying the elusive substance. The discovery of a dark matter “bullet” would also fit with current predictions about what dark matter is like—research suggests it is “clumpy,” in that it is not smooth and evenly distributed around the universe.

Identifying a clump of dark matter “opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures,” her abstract concludes.

The first accurate 3D map of our galaxy revealed its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy. The artist’s impression, Chen Xiaodian, of the warped and twisted Milky Way disk is shown below.

An international team of astronomers discovered that the Milky Way’s disc of stars becomes increasingly ‘warped’ and twisted the further away the stars are from the galaxy’s center. “We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope,” says Professor Richard de Grijs, an astronomer from Australia’s Macquarie University.

Image credit top of page: ESA Advanced Concepts

The Daily Galaxy, Sam Cabot, via Ana Bonaca, Dynamical Evidence for a Dark Substructure in the Milky Way Halo UCSB, and Live Science

Originally posted on May 16, 2019 (122,784 views)