The answer is in the intergalactic wind

Astronomers have observed the first direct evidence that an intergalactic “wind” is stripping galaxies of star-forming gas as they fall into clusters of galaxies. This explains why galaxies found in clusters have relatively little gas and less star formation compared to non-cluster or “field” galaxies. It was thought that as a field galaxy falls into a cluster of galaxies, it encounters a cloud of hot gas. As it moves through this intra-cluster medium, the cloud acts like a wind to blow away the gas within the galaxy without disturbing its stars - a process known as ram-pressure stripping. Astronomers had previously seen the very tenuous atomic hydrogen gas surrounding a galaxy get stripped, but believed the denser molecular hydrogen clouds where stars form would be more 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 Suresh Sivanandam of the University of Toronto.” The scientists studied four galaxies, one of which had been stripped of its star-forming gas by this wind, shown as a wake trailing from the galaxy in the direction opposite to its motion. By observing four galaxies, they have now shown that this effect is common. The team used optical, infrared and hydrogen-emission data from the Spitzer and Hubble space telescopes, plus archival ground-based data. “Seeing this stripped molecular gas is like seeing a theory on display in the sky,” said Marcia Rieke of 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.” “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 George Rieke, also from Arizona. “Cutting off the gas that forms stars is a key step in the evolution of galaxies from the early Universe to the present.” Tracing ram-pressure stripping with warm molecular hydrogen emission