Astronomers Catch Wind Rushing Out of Galaxy

Exploring the influence of galactic winds from a distant galaxy called Makani, UC San Diego’s Alison Coil, Rhodes College’s David Rupke, and a group of collaborators from around the world made a novel discovery. Published in Nature, their study’s findings provide 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. The unique composition of Makani — meaning wind in Hawaiian — uniquely lent itself to the breakthrough findings. Coil, a physics professor at UC San Diego, said:

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, noting that:

Coil and 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.

Figure 1: The giant galactic wind surrounding the massive, compact galaxy Makani. The colors and white contour lines show the amount of light emitted by the ionized gas from different parts of the oxygen nebula, from brightest (white) to faintest (purple). The middle part of the image (black) shows the full extent of the galaxy, though most of the galaxy is concentrated at the center (the tiny green circle). The axes show distance from the center of the galaxy in kiloparsecs. (Image: Gene Leung, UC San Diego)

The giant galactic wind surrounding the massive, compact galaxy Makani. The colors and white contour lines show the amount of light emitted by the ionized gas from different parts of the oxygen nebula, from brightest (white) to faintest (purple). The middle part of the image (black) shows the full extent of the galaxy, though most of the galaxy is concentrated at the center (the tiny green circle). The axes show distance from the center of the galaxy in kiloparsecs. (Image: Gene Leung, UC San Diego)

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. Rupke, associate professor of physics at Rhodes College, summarized by saying:

From 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.

Figure 2: The multiphase galactic wind: comparison of the ionized, neutral atomic and molecular gas. In the zoomed-in view of the inner 40 kiloparsecs at the upper right, molecular gas emission from carbon monoxide (green contours) is plotted on emission from magnesium atoms that trace neutral atomic gas (color, with white contours) in the same velocity range (-500 to +500 kilometers per second, where negative velocities are blueshifted and positive velocities redshifted with respect to the galaxy). The zoomed-in view at the lower left compares the emission from low-velocity molecules and ionized oxygen atoms, and the high-velocity molecular and ionized gas are shown at lower right. The molecules, neutral atoms and ionized gas all correspond well spatially, though the ionized gas extends far beyond the other two gas phases. (Image: David Rupke, Rhodes College)

The multiphase galactic wind: comparison of the ionized, neutral atomic and molecular gas. In the zoomed-in view of the inner 40 kiloparsecs at the upper right, molecular gas emission from carbon monoxide (green contours) is plotted on emission from magnesium atoms that trace neutral atomic gas (color, with white contours) in the same velocity range (-500 to +500 kilometers per second, where negative velocities are blueshifted and positive velocities redshifted with respect to the galaxy). The zoomed-in view at the lower left compares the emission from low-velocity molecules and ionized oxygen atoms, and the high-velocity molecular and ionized gas are shown at lower right. The molecules, neutral atoms and ionized gas all correspond well spatially, though the ionized gas extends far beyond the other two gas phases. (Image: David Rupke, Rhodes College)

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. Coil noted:

Rupke noticed that the hourglass shape of Makani’s nebula is strongly reminiscent of similar galactic winds in other galaxies, but that Makani’s wind is much larger than in other observed galaxies. Rupke explained, saying:

Rupke also noted that while astronomers are converging on the idea that galactic winds are important for feeding the CGM, most of the evidence has come from theoretical models or observations that don’t encompass the entire galaxy, saying:

Provided by: University of California – San Diego [Note: Materials may be edited for content and length.]

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