Celestial cannonball

The spiral galaxy appearing in this week’s Hubble Picture of the Week is named IC 3225. It looks remarkably as if it’s been launched from a cannon, speeding through space like a comet with a tail of gas streaming from its disc behind it. The scenes that galaxies appear in from Earth’s point of view are fascinating; many seem to hang calmly in the emptiness of space as if hung from a string, while others star in much more dynamic situations!

Appearances can be deceiving with objects so far from Earth — IC 3225 itself is about 100 million light-years away — but the galaxy’s location suggests some causes for this active scene, because IC 3225 is one of over 1300 members of the Virgo galaxy cluster. The density of galaxies in the Virgo cluster creates a rich field of hot gas between them, the so-called ‘intracluster medium’, while the cluster’s extreme mass has its galaxies careening around its centre in some very fast orbits. Ramming through the thick intracluster medium, especially close to the cluster’s centre, places an enormous ‘ram pressure’ on the moving galaxies that strips gas out of them as they go.

IC 3225 is not so close to the cluster core right now, but astronomers have deduced that it has undergone this ram pressure stripping in the past. The galaxy looks as though it’s been impacted by this: it is compressed on one side and there has been noticeably more star formation on this leading edge, while the opposite end is stretched out of shape. Being in such a crowded field, a close call with another galaxy could also have tugged on IC 3225 and created this shape. The sight of this distorted galaxy is a reminder of the incredible forces at work on astronomical scales, which can move and reshape even entire galaxies!

[Image Description: A spiral galaxy. Its disc glows visibly from the centre, and has faint dust threaded through it. A spiral arm curves around the left edge of the disc and is noticeably more dense with bright blue spots, where there are hot and new stars, than the rest. Opposite, the disc stretches out into a short tail where it covers a distant background galaxy. Around it, other distant galaxies and some nearby stars are visible.]