Galactic gas snapped
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Scientists have proven that galaxies are much much bigger than they appear.
A shroud of gas stretches up to a million light years around every galaxy and is its first interaction with the wider Universe beyond.
For the first time, scientists have photographed this halo of matter and examined it pixel by pixel. The findings suggest that the Milky Way may already be mingling with its closest neighbour, Andromeda.
The circumgalactic medium (CGM), a vast cloud of gas surrounding a galaxy, plays a pivotal role in defining where a galaxy ends and space begins.
“We found it everywhere we looked, which was really exciting and kind of surprising,” says Associate Professor Nikole M Nielsen, lead author of the study and a researcher at Swinburne University and ASTRO 3D.
The research, which used the Keck Cosmic Web Imager (KCWI) on the 10-metre Keck telescope in Hawaii, observed a star-bursting galaxy 270 million light years away, mapping the CGM out to 100,000 light years from the galaxy’s disc.
In comparison, the visible stellar disc of the galaxy extends only about 7,800 light years.
The study provides the first detailed observation of the transition between a galaxy’s inner region and the surrounding medium.
“We’re now seeing where the galaxy's influence stops, the transition where it becomes part of more of what’s surrounding the galaxy, and, eventually, where it joins the wider cosmic web and other galaxies,” Dr Nielsen explained.
The circumgalactic medium, which comprises about 70 per cent of the galaxy’s mass (excluding dark matter), has long been difficult to observe.
Previously, scientists could only infer its presence by measuring how light from distant objects, such as quasars, was absorbed as it passed through the gas.
However, the KCWI instrument’s integral field spectrograph allowed the researchers to capture thousands of spectra in one image, enabling a full view of the CGM’s structure and properties.
One key finding is the physical connection of elements like hydrogen and oxygen from the centre of the galaxy into the outer reaches of space.
This connection alters the physical conditions of the gas as it moves further from the galaxy.
“In the CGM, the gas is being heated by something other than typical conditions inside galaxies,” Dr Nielsen says.
The heating likely comes from the collective emissions of other galaxies and possibly from shock waves, indicating that galaxies do not exist in isolation but are influenced by their environment.
This discovery opens new avenues for understanding how galaxies evolve, particularly in terms of how they acquire, process, and expel gas.
The study also sheds light on the CGM’s role in this gas cycle, which could drive significant changes within galaxies.
The findings have broader implications for understanding galactic interactions.
“It’s highly likely that the CGMs of our own Milky Way and Andromeda are already overlapping and interacting,” Dr Nielsen said.