A collision of two galaxies caused the unusual appearance of the Blackeye Galaxy. The black area in this galaxy is due to massive amounts of dust on this side of the center. While the stars are all revolving around the galaxy’s center in the same sense, a significant amount of gas in the outer regions is moving in the opposite sense. From this, astronomers deduce that a smaller galaxy collided with M64 in the past, perhaps a billion years ago or more.
There is a boundary where the outer region gas, moving in the opposite sense of the stars, meets gas in the inner region, which moves with the stars. In the boundary region the gas clouds moving in opposite directions are colliding, leading to higher density regions, and new star formation is thus enhanced.
The pink regions are hydrogen gas glowing in the red part of the spectrum after absorbing ultraviolet light from new hot stars.
Image credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)