The Intergalactic Winds That Built the Milky Way

Computer simulations show that powerful streams of gas can leave one galaxy, travel across space, and get absorbed into another.

Galaxies M81 and M82 in the constellation Ursa Major
Galaxies M81 and M82 in the constellation Ursa Major (Courtesy of Fred Herrmann)

Even though there’s no air blowing around in space, the cosmos can be a pretty windy place.

Winds made of gas particles whip around galaxies at high speeds, measuring hundreds of kilometers per second. Astronomers suspect they’re created by supernovae, when nearby stars explode and send streams of photons powerful enough to push around gas. This wind can be ejected out of galaxies into intergalactic space. Some of the wind will keep traveling into the void, while a fraction will get sucked back in—a process known as wind recycling.

While astronomers have known for decades that galactic winds exist, they’re still trying to determine precisely what triggers and drives them. Daniel Anglés-Alcázar, an astrophysicist at Northwestern University in Illinois, and his colleagues recently decided to study the process behind wind recycling. They plugged a bunch of data into computer simulations of galaxy formations, ready to compare their results with previous models.

They got a big surprise instead. The simulations showed that supernovae, the explosions of dying stars, can produce winds of gas that can escape their parent galaxy, travel for billions of years across intergalactic space, and become absorbed into a different galaxy, where the newly arrived atoms may form new stars. The simulations demonstrated that gas flows from smaller galaxies to larger galaxies, like the Milky Way. The researchers dubbed the phenomenon, which Anglés-Alcázar said hasn’t been studied before, intergalactic transfer.

Based on the simulations, the researchers conclude that intergalactic winds may be responsible for up to 50 percent of matter in the larger galaxies. Half of our very own galaxy, they say, could have originated from another corner of the universe. The atoms that built the solar system, the planets, and even our own bodies may have come from another galaxy perhaps as far as 1 million light-years away.*

“It was actually surprising for us to find out that there is that much exchange of mass between galaxies,” Anglés-Alcázar said. “When something comes out of the data, out of the simulation, that you do not expect, then definitely that implies that you need to go deep into the data and compute the same result in many different ways and to be 100 percent sure that there is nothing going wrong with the result.”

The researchers converted the simulations into three-dimensional visuals, which Anglés-Alcázar said are “very much needed to convince yourself that what comes out of the calculation makes sense.” Their findings were published Wednesday in the Monthly Notices of the Royal Astronomical Society.

Here’s a few examples of intergalactic winds, shown as green string, in action around galaxies, shown as clusters of yellow stars. The galaxy at the center is absorbing winds ejected by other galaxies:

Daniel Anglés-Alcázar / Northwestern University

And here’s an animation of gas flows around a Milky Way-like galaxy, as seen by the team’s computer simulations:

Philip Hopkins / Caltech

Anglés-Alcázar said more computer-simulation experiments are needed to better understand intergalactic transfers. The phenomenon could perhaps even be directly observed using Hubble, the galaxy-hunting space telescope. Astronomers could direct Hubble’s mirror toward quasars, the brightest objects in the universe that shine with the intensity of 100 galaxies. Gas surrounding a galaxy in the line of sight could absorb some of a quasar’s light. Astronomers could study the wavelengths that gas absorbs and reflects to get some clues about its properties.

Other computer-simulation research on galaxies has shown that the Milky Way’s contents may have extragalactic origins. In January, Harvard-Smithsonian Center for Astrophysics astronomers found that some of the most distant stars in the galaxy were pulled from the Sagittarius dwarf galaxy, one of the dozens of smaller galaxies orbiting our cosmic neighborhood. Such research illustrates the subtle fluidity of massive cosmic objects that appear, from a great distance in telescope images, as static, glittering gems. From our vantage point, galaxies often look like unmoving clusters of stars that have been minding their own business since the Big Bang billions of years ago. But they are dynamic structures, some apparently with a penchant for donating and borrowing some star-making dust from their neighbors.

“It’s very interesting to think of our galaxy not as some isolated entity, but to think of the galaxy as being surrounded by gas which may come from many different sources,” Anglés-Alcázar said. “We are connected to other galaxies via these galactic winds.”

* This article originally misstated the distance intergalactic winds may travel as one light-year. We regret the error.

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