Intergalactic Winds Vital for Galaxy Evolution

Galactic winds are the result of the explosion of massive stars; they allow the exchange of matter between galaxies and their surroundings and play a critical role in the evolution of galaxies by regulating their mass build-up and star formation

For the first time, an international team of astronomers have observed in three dimensions that gas from spiral galaxies is blown upward and downward at high velocity, far out of the galaxy, confirming the theory of galaxy evolution in which star-forming galaxies drive intergalactic winds by venting their gas along the poles.

Although this had already been observed in the local universe, this phenomenon has been seen in galaxies which are over 7 billion years old and actively forming stars.

Galactic hide and seek

Galactic winds, or outflows, are important in models describing the formation of galaxies; but they are diffuse and of low gas density and low surface brightness – meaning they can be hard to spot.

Galaxies grow by seizing gas from their surroundings; it’s thought this growth is repressed by young stars and supermassive black holes, which blow gas out into space through shock waves. Exactly what happens is unknown, but without strong gas flows, galaxies could become far too big.

Researchers led by Yucheng Guo of the Lyon Astrophysical Research Centre and the National Centre for Scientific Research (CNRS) combined images of over a hundred galaxies taken over very long exposure times by Multi-Unit Spectroscopic Explorer (MUSE), an instrument integrated into the European Southern Observatory’s (ESO) Very Large Telescope.

Magnesium and morphology

To view the galactic winds, researchers studied magnesium atom emission signals from almost two hundred distant spiral galaxies; this enabled them to map the morphology of the winds. In half of these galaxies, they observed the disk edge-on, and in the other half, the disk was face-on and appeared as a circle.

In the edge-on galaxies, the gas outflow was perpendicularly upward and downward. "We detect the gas up to tens of thousands of light years from the galaxy, moving through intergalactic space at hundreds of kilometers per second," says Yucheng Guo, first author of the Nature study.

"To me, it is a real milestone that we are finally witnessing intergalactic gas outflows from ordinary galaxies," says co-author Joop Schaye of Leiden University. "Until now, observations have been difficult to interpret, but thanks to this study we can no longer ignore bipolar winds."

Now they have mapped average gas flows and velocities, the researchers can test and adjust their computer simulations of galaxy evolution to clarify how galaxies grow. They also hope to measure how far these winds extend and the quantity of matter they transport.

" We still don't know much about their physical properties, such as their size and strength, or how they vary with time and in different types of galaxies," says Guo.
"Answering these questions will require deeper observations and next-generation facilities such as the BlueMUSE instrument on the VLT and the Wide-field Spectroscopic Telescope (WST)."