M99 (NGC 4254) was discovered on March 15, 1781 by Messier's colleague and friend, Pierre Méchain. Charles Messier measured its position and included it in his catalog on April 13, 1781, immediately before finishing the third and final published edition.
M99 is situated in the constellation Coma Berenices approximately 50 million light-years from the Earth. It is a member of the Virgo Cluster of galaxies. M99 is classified as a type Sc galaxy but is markedly asymmetrical with one of its main spiral arms ‘pulled’ to one side. One hypothesis for the distorted shape of M99 is a gravitational interaction with a theoretical ‘dark galaxy’. A bridge of gas links M99 to a large neutral hydrogen gas cloud designated VIRGOH121. Studies indicate a large mass of dark mater equivalent to that of a small galaxy lies within the cloud. It is conjectured this mass is the ’dark galaxy’ thought to be the source of the distorting influence.
A high resolution image, processed from Hubble Legacy Archive data, of the core of M99 may be viewed here.