Astronomers Are Hunting for a ‘Hidden’ Halo Orbiting the Milky Way

Ever since it was first observed in 1965, the Magellanic Stream—a massive river of gas and galaxy guts that orbits the Milky Way at a distance of roughly 180,000 light years—has puzzled scientists.

The stream, which stretches for an astonishing 600,000 light years and is about a billion times as massive as the Sun, is clearly fed by two smaller galaxies called the Small and Large Magellanic Clouds, which were captured by the Milky Way’s gravity eons ago.

While outflows from these galaxies explain some of the Magellanic Stream’s properties, no models have been “able to provide a full understanding of its origins,” particularly its colossal billion-Sun mass, according to a paper published on Wednesday in Nature. The new research bridges this gap by proposing that the stream is enveloped by an as-yet undetected halo of warm gas, which the paper calls the “Magellanic Corona.”

“What people have thought is that this gas is like an outflow, like a galactic rain,” said Elena D'Onghia, a professor of astronomy at the University of Wisconsin-Madison and one of the authors of the paper, in a call. “We believe that there must be a warm corona that has been hidden so far.”

D'Onghia and her colleagues—including UW-Madison graduate student Scott Lucchini, who led the research, as well Space Telescope Science Institute astronomer Andrew Fox—came to this conclusion by running simulations that combined the traditional models of tidal outflows from the Magellanic Clouds with the presence of an undiscovered corona made of ionized warm gas. The team’s models shed light on many unresolved mysteries about the huge structure, including its hulking mass and the curious lack of stars embedded in its extent.

“In tidal models, stars (in addition to gas) should be stripped from both clouds as a result of the gravitational interactions they experience before falling into the Milky Way,” the researchers explained in the paper. “Such a stellar stream has yet to be discovered, even though sensitive searches have been conducted.”

“However, in our model, the stream is formed mostly by the warm Magellanic Corona, so its stellar counterpart is negligible,” the team said.

This speculative corona has never been spotted because nobody has ever sought it out. D'Onghia and her colleagues already have a plan in place to detect it with the help of distant quasars, which are energetic galaxies located billions of light years from the Milky Way.

If the Magellanic Stream really is coated in a warm corona, the light from these extraordinarily bright quasars will filter through it, leaving observable traces that the researchers hope to capture within the next few months.

“We’ve already got time on the Hubble Space Telescope, so we are going to look at the data,” said D'Onghia. “In 2021, we should be able to unambiguously detect the corona, if it exists.”