A vast wind caused by the passage of a galaxy through hot gas has been observed eroding the galaxy's leading edge and shaping dust formations on an almost unimaginable scale.

Outer space is not truly empty, and while the voids between galaxies may have only 1,000 particles per cubic meter (30 per cubic foot), that can still be enough to create mighty winds that sculpt galactic outlines. 

We see something similar within our own galaxy, such as the columns of dust dubbed the “Pillars of Creation,” where the stellar winds from giant stars sculpt the clouds of gas and dust around them, interfering with star formation. When galaxies run into the hot gas within clusters, the same thing was suspected to happen many times larger.

When Professor Jeffrey Kenney of Yale University examined Hubble Space Telescope images of galaxy NGC 4921, he could see the process in action with astonishing clarity. "On the leading side of the galaxy, all the gas and dust appears to be piled up in one long ridge, or dust front. But you see remarkable, fine scale structure in the dust front," said Kenney.

^{Extraordinary detail and hot blue stars can be seen at NGC 4921's leading edge. Credit: NASA, ESA, and Roberto Colombari}

NGC 4921 is the most massive spiral galaxy in the Coma cluster, a collection of galaxies with a combined mass 1,000 times greater than the Milky Way. The Coma cluster lies 300 million light-years from Earth and is 10 times the mass of any of the closer clusters. As such, everything is bigger there, including the speed at which galaxies move and the density of the gas between them. The filaments Kenney described are at least 1,000 times the size of the Pillars of Creation, which is why we are able to see them at such a distance. 

NGC 4921’s dust front stretches for more than 60,000 light-years, or around half the length of the Milky Way, Kenney reports in The Astrophysical Journal. The structures seen within it, which the paper describes as including “head-tail filaments, C-shaped filaments and long smooth dust fronts,” are between 300 and 3,000 light-years across. In many cases, the heads of these shapes feature collections of huge, bright stars.

^{Contour lines of hydrogen within NGC 4921 showing the way it has bunched up at the leading edge and spread at the other end. Kenney et al. in arXiv.org}

Like rock formations in Monument Valley, which they somewhat resemble, the pillars represent relatively dense material that has survived while erosion wears away at what surrounds them. The authors note that something seems to be acting to hold parts of the interstellar medium together against this erosion, and speculate it may be magnetic fields.

Eventually, even the dust will fall before the wind, dispersing and preventing new stars from forming at NGC 4921’s leading edge.

In smaller galactic clusters, we see similar impacts on dwarf galaxies, but formations the size of NGC 4921 are less affected, the paper notes. “A great deal of galaxy evolution is driven by interactions," Kenney said. "Galaxies are shaped by collisions and mergers, as well as this sweeping of their gas from cosmic winds."