A group of astronomers have found 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," reports Phys.Org. "The unique composition of Makani -- meaning wind in Hawaiian -- uniquely lent itself to the breakthrough findings." From the report: "Makani is not a typical galaxy," noted [UC San Diego's Alison 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. Coil and [Rhodes College's David Rupke], the paper's first author, used data collected from the W. M. Keck Observatory's new Keck Cosmic Web Imager (KCWI) instrument, combined with images from the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA), to draw their conclusions. 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 year ago, from a gas outflow launched hundreds of millions of years earlier that has since slowed significantly. From the Hubble, the researchers procured images of Makani's stars, showing it to be a massive, compact galaxy that resulted from a merger of two once separate galaxies. From ALMA, they could see that the outflow contains molecules as well as atoms. The data sets indicated that with a mixed population of old, middle-age and young stars, the galaxy might also contain a dust-obscured accreting supermassive black hole. This suggests to the scientists that Makani's properties and timescales are consistent with theoretical models of galactic winds. The findings have been published in the journal Nature.
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