A newly released set of images, taken by the NASA/ESA Hubble Space Telescope before the recent Servicing Mission, highlights the ongoing drama in two galaxies in the Virgo Cluster affected by a process known as "ram pressure stripping" that can result in peculiar-looking galaxies. An extremely hot X-ray emitting gas known as the intra-cluster medium lurks between galaxies within clusters. As galaxies move through this intra-cluster medium, strong winds rip through galaxies distorting their shape and even halting star formation.
Ram pressure is the drag force that results when something moves through a fluid — much like the wind you feel in your face when bicycling, even on a still day — and occurs in this context as galaxies orbiting about the center of the cluster move through the intra-cluster medium, which then sweeps out gas from within the galaxies.
The spiral galaxy NGC 4522 is located some 60 million light-years away, and it is a spectacular example of a spiral galaxy being stripped of its gas content. The galaxy is part of the Virgo galaxy cluster, and its rapid motion within the cluster results in strong winds across the galaxy as the gas within is left behind. Scientists estimate that the galaxy is moving at more than 6.2 million miles (10 million kilometers) per hour. A number of star clusters that formed in the stripped gas can be seen in the Hubble image.
Hubble's view of NGC 4522 highlights the dramatic state of the galaxy, with an especially vivid view of the ghostly gas being forced out of it. Bright blue pockets of new star formation can be seen to the right and left of center. The image is sufficiently deep to show distant background galaxies.
The image of NGC 4402 also highlights some telltale signs of ram pressure stripping such as the curved, or convex, appearance of the disc of gas and dust, a result of the forces exerted by the heated gas. Light being emitted by the disk backlights the swirling dust that is being swept out by the gas. Studying ram pressure stripping helps astronomers better understand the mechanisms that drive the evolution of galaxies, and how the rate of star formation is suppressed in dense regions of the universe — like clusters.