Astronomers Catch Wind Rushing Out of Galaxy

October 31, 2019, 10:23 AM HST (Updated October 31, 2019, 11:15 AM)
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Astronomers using the W.M Keck Observatory on Maunakea observed a galaxy eject huge amounts of gas into space.

Galactic winds like this have only been seen through computer models – until now, according to a press release from W.M. Keck Observatory released today. Direct evidence of the giant galactic wind surrounding the galaxy named Makani (wind) was captured using one of the observatory’s newest instruments, the Keck Cosmic Web Imager.

A volume rendering of the ionized gas wind in Makani. Two of the dimensions are
spatial, and the third is velocity. The colors trace the velocity axis, shown by the arrow
at center. The approximate locations of the two proposed outflow episodes are labeled. (PC: Jim Geach, David Tree, Peter Richardson of University of Hertfordshire)

The KCWI data provided what the researchers call the “stunning detection” of the ionized oxygen gas to extremely large scales, well beyond the stars in the galaxy. It allowed them to distinguish a fast gaseous outflow launched from the galaxy a few million years ago, from a gas outflow launched hundreds of millions of years earlier that has since slowed significantly.

University of California, San Diego’s Alison Coil, Rhodes College’s David Rupke, and a group of collaborators published their findings in the journal, Nature. Their study shows direct evidence for the first time of the role of galactic winds—ejections of gas from galaxies—in creating the circumgalactic medium (CGM). It exists in the regions around galaxies, and it plays an active role in their cosmic evolution.

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“Makani is not a typical galaxy,” stated Coil, a physics professor at UC San Diego. “It’s what’s known as a late-stage major merger—two recently combined similarly massive galaxies, which came together because of the gravitational pull each felt from the other as they drew nearer. Galaxy mergers often lead to starburst events, when a substantial amount of gas present in the merging galaxies is compressed, resulting in a burst of new star births. Those new stars, in the case of Makani, likely caused the huge outflows—either in stellar winds or at the end of their lives when they exploded as supernovae.”

Coil explained that most of the gas in the universe inexplicably appears in the regions surrounding galaxies—not in the galaxies. Typically, when astronomers observe a galaxy, they are not witnessing it undergoing dramatic events—big mergers, the rearrangement of stars, the creation of multiple stars or driving huge, fast winds.

“While these events may occur at some point in a galaxy’s life, they’d be relatively
brief,” Coil stated. “Here, we’re actually catching it all right as it’s happening through
these huge outflows of gas and dust.”

Coil and Rupke used data collected from one of Keck Observatory’s newest instruments – the Keck Cosmic Web Imager (KCWI) – combined with images from the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA), to draw their conclusions.

“The earlier outflow has flowed to large distances from the galaxy, while the fast, recent
outflow has not had time to do so,” stated Rupke, associate professor of physics
at Rhodes College.

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