Like most galaxies, the ‘Sombrero’ galaxy is highly symmetrical on all three axes.
Credit: STScI/NASA

An asymmetrical galaxy usually results when one galaxy gravitationally interacts with another through either a ‘fly-by’ or a merger event. This interaction disrupts the galaxy (galactic disks are particularly sensitive to such disturbances), and often triggers a burst of new star formation. For this reason, asymmetrical galaxies are typically disk galaxies with high rates of star formation.

Models suggest that once the disturbing influence has gone (i.e. once the fly-by or merger is completed) galaxies settle back to a symmetrical configuration in about 500 million years, and star formation returns to a more normal rate. This makes asymmetrical galaxies transient and relatively rare objects. Even so, asymmetries in galaxies, along with warps, tidal tails and shells provide astronomers with a means to probe both ongoing and recently completed interactions, as well as the dynamics of the galaxies involved.

NGC772 (Arp78) is surrounded by a swarm of satellite dwarf galaxies (fuzzy blobs). Its asymmetry was caused through an interaction with one of these satellite galaxies (the bright one immediately above NGC772). Credit: T. Credner & S. Kohle, AlltheSky.com

The asymmetry in the disk of NGC 4319 is the result of an interaction with another galaxy (NGC 4291, not shown). Credit: NASA and The Hubble Heritage Team (STScI/AURA)

The clearly asymmetrical NGC 6745 has a trail of blue stars leading to the lower right of this image. This is a trail of new-born stars created by the passage of a small galaxy (just visible at bottom right). Credit: NASA and The Hubble Heritage Team (STScI/AURA)