EYE ON THE SKY

Unusual, unexpected, unexplained and exciting… mysterious phenomena provide astronomers with more clues about the workings of the Universe

The classic image of a spiral galaxy is of a crisp, symmetrical structure with two arms swirling around a bright core. NGC 4625 bucks the galactic trend as it has just one spiral arm, giving it an odd asymmetry.

This galaxy is about 30 million lightyears away in the constellation of Canes Venatici, close to a dwarf galaxy called NGC 4618. Astronomers are studying NGC 4625 to try and work out why it has such an unusual shape. Could the galaxy have formed this way, or did something happen to it that caused it to lose one of its arms? One theory is that NGC 4625 came into close contact with its nearby dwarf companion at some point in the past, and the interacting gravitational forces distorted its shape.

NGC 4625’s disc appears to be four times larger when viewed in ultraviolet light than in the optical light image you see here. This indicates that there is a large number of young, hot stars forming in its outer regions. At about one billion years old, these stars are 10 times younger than the stars in the optical centre.

This age differential gives astronomers a clue as to how NGC 4625 lost one of its arms, as galactic interactions are known to trigger bursts of star formation. NGC 4618 could indeed be the culprit. It may have come too close to NGC 4625 and torn off one of its arms, triggering the formation of a new batch of stars in the process.