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Astronomers have found evidence that intergalactic 'wind' is stripping certain galaxies of the gas they need to form stars.

The process was spotted in four galaxies as they fell into dense clusters of other galaxies.

And the results could explain why some galaxies in clusters have so few young stars compared to others, like our own Milky Way.

Astronomers from the University of Toronto and the University of Arizona have found direct evidence for galaxies being stripped of star-forming gas. Pictured is one of the observed galaxies, NGC 4522. The process occurs when a previously 'lonely' galaxy falls into a cluster

The research was carried out by astronomers from the University of Toronto and the University of Arizona using Nasa's Spitzer Space Telescope and the Hubble Space Telescope.

It was previously suggested that so-called 'field galaxies' - those drifting through the universe alone - would have their gas stripped away if they moved into a galaxy cluster.

And these results are the first time direct evidence for the theory that has been found.

The effect occurs as the previously single galaxy encounters a cloud of hot gas at the centre of the cluster.

As it moves through this region at thousands of miles per second, the cloud acts as a wind and blows away the gas within the galaxy without disturbing its stars.

The process is known as ram-pressure stripping.

Previously, astronomers had seen the very tenuous atomic hydrogen gas, that surrounds a galaxy, get stripped.

But it was thought these denser, molecular hydrogen clouds, where stars can form, were resistant to the wind.

'However, we found that the molecular hydrogen gas is also blown from the in-falling galaxy, much like smoke blown from a candle being carried into a room,' said Dr Suresh Sivanandam of the Dunlap Institute at the University of Toronto.

Amazingly, astronomers had previously spotted young stars lying in the wake of such field galaxies - providing indirect evidence for the ram-pressure stripping process.

But now they have seen direct evidence of the molecular hydrogen itself, trailing in the wake of several galaxies. 

The research was carried out by astronomers using Nasa's Spitzer Space Telescope (illustration shown) and the Hubble Space Telescope. It was previously suggested that so-called 'field galaxies' would have their gas stripped away but this is the first direct evidence

This image shows one of the observed galaxies, ESO 137-001, with a stream of gas trailing behind it. It's believed that stars can actually form in this stream of material behind the galaxy as it heads through the centre of a cluster of other galaxies

'For more than 40 years we have been trying to understand why galaxies in dense clusters have so few young stars compared with ones like our Milky Way Galaxy, but now we see the quenching of star formation in action,' said Professor George Rieke of the University of Arizona.

'Cutting off the gas that forms stars is a key step in the evolution of galaxies from the early universe to the present.'

The results, published in the Astrophysical Journal, are from observations of four galaxies - NGC 4522, CGCG 97-073, ESO 137-001 and NGC 1427a.

The team made their analysis using optical, infrared and hydrogen-emission data from the Spitzer and Hubble space telescopes, as well as ground-based data. 

'Seeing this stripped molecular gas is like seeing a theory on display in the sky,' said Professor Marcia Rieke, also from the University of Arizona.

'Astronomers have assumed that something stopped the star formation in these galaxies, but it is very satisfying to see the actual cause.'

This remarkable image shows MACS J0152.5-2852 in the middle, a massive galaxy cluster not associated with this study. Almost every object seen in the image is a galaxy, each containing billions of stars