Ram pressure stripping has been caught in the act in images of the Virgo Cluster snapped by the Hubble Space Telescope prior to its servicing mission earlier this year.

Ram pressure is the drag force that results when something moves through a fluid, rather like the wind you feel in your face when bicycling on a still day. In space, galaxies move through the intra-cluster medium, an extremely hot X-ray emitting gas that lurks between galaxies and within clusters. As galaxies move through the intra-cluster medium, strong winds rip through the galaxies, distorting their shape by sweeping out gas, and even switching off star formation.

The newly released images show this drama unfolding within two galaxies in the Virgo Cluster – NGC 4522 and NGC 4402. Spiral galaxy NGC 4522 appears as a swirling whirlpool of gas and dust and is an archetypal example of a galaxy being stripped of its gas. As it races through the cluster at some 10 million kilometres per hour strong winds whip across the galaxy, forcing ghostly tendrils of gas and dust out of its clutches.

A number of star clusters that subsequently formed in the stripped gas can be seen in the Hubble image as bright blue pockets of new stars to the right and left of centre. Distant background galaxies are also visible in the image.

Tell-tale signs of ram pressure stripping are also revealed in the snapshot of NGC 4402. The curved, convex appearance of the galaxy's disc of gas and dust is a result of the forces exerted by the heated gas of the intra-cluster medium. Light emitted by the disc backlights the swirling dust that is being swept out by the gas.

Studying the effects of ram pressure stripping helps astronomers learn about the mechanisms that drive the evolution of galaxies, and how the rate of star formation is suppressed in very dense regions of the Universe like clusters.

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