Coma Pinwheel Galaxy

A spiral galaxy that forms part of the huge Virgo Cluster, M99 falls on the western side of the Cluster as seen from Earth, within the boundaries of Coma Berenices slightly to the north of that constellation's boundary with Virgo.

M99 is roughly fifty million light years from the Milky Way Galaxy, and angled such that it appears almost face-on to an observer on Earth, with its spiral structure clearly visible. Indeed, it was one of the first galaxies whose spiral form was recognised (the galaxy was discovered in 1781, but it was not understood to be a spiral until sixty-five years later in 1846).

A striking feature of M99 is that its spiral shape is not symetrical: its long northern arm is wound much less tightly than the other arms, angling loosely away from the body of the galaxy. This is due to the gravitational effects within the Virgo Cluster, but the precise cause is unclear. This may simply be due to a historical interaction, for example with nearby M98, but an alternative hypothesis suggests that the distortion is due to a so-called 'dark galaxy'. This suggests the presence of a galaxy composed entirely of dark matter, detectable only by its gravitational effects on nearby objects such as M99.