The Black Eye Galaxy : An unbelievable mystery

The Black Eye Galaxy : M 64

Credits: NASA and the Hubble Heritage Team (AURA/STScI); Acknowledgment: S. Smartt (Institute of Astronomy) and D. Richstone (U. Michigan)

Galaxy M64 or Black Eye Galaxy as we call it, is a strange or rather peculiar galaxy and it has been this way for the past billion of years, like the galaxy itself, its discovery too keeps surprising the astronomers. In 2002 we came to know that it was actually discovered by English astronomer Edward Pigott on 23 March 1779 just 12 days before German astronomer Johann Bode and a year before French astronomer Charles Messier rediscovered it and added it to his famous catalog as Messier 64. Pigott’s discovery remained unknown until it was read before the Royal Society of London in 1781 and surprisingly this fact only came to be known in 2002. Unfortunately we know next to nothing about Edward Pigott despite the fact that he also discovered the first Cepheid variable stars that help astronomers uncover cosmic distances.

Star chart for M64 represents view from mid-northern latitudes

Credits: Image courtesy of Stellarium

M 64 or Black Eye galaxy is located in the constellation of Coma Berenices, Its peculiar structure and appearance soon became obvious when William Herschel made several observations of M 64 after witnessing enormous blackness enveloping a huge part of this spiral galaxy he named it Black Eye Galaxy. Which remains in use to this day, thus M64 became the first and only celestial object named after an injury.

In 1948 astronomers discovered that the black feature is a huge complex cloud of carbon particles surrounding a bright inner zone, while spectroscopy later revealed the strange or rather odd waves of star formation that over the passage of time jumped from one section of the black eye galaxy to another. The newest burst of  formation only now seems to be reaching the vast dusty region, which resultantly should start coming alive with bright blue specks in later epochs.

M 64 : The Black Eye Galaxy

Black Eye Galaxy does not stop to surprise just yet. Measurements of the motions of its stars show that, unlike normal spiral galaxies that rotate in a single direction, Black Eye Galaxy is made of counter rotating segments. The inner 3,000 light year radius segment spins clockwise, while the galaxy’s remaining outer 40,000 light years wide segment moves in the other direction. 

ARP 87  pair of two galaxies, and an example of a galactic collision
Credit : NASA / ESA / AURA

This odd rotation of Black Eye Galaxy is hard to explain. There is only one viable explanation that in the past, probably a billion years ago, Black Eye Galaxy collided with another galaxy. The two galaxies merged into one while still maintaining their separate segments that rotate in different directions to this day. The shearing zone where the two opposing gaseous segments collide  against each other remains an extremely turbulent region that instigates several nebulas and new star production. The huge dark and incongruous dust cloud is leftover material from M64’s former companion. Although it’s been pulled in, it hasn't yet had time to fully merge with the plane of the main galaxy’s disk.

Now this odd looking galaxy surprises us once again. Another oddity is that we do not know how far away the Black Eye Galaxy lies from us. Though It would be scientifically rewarding to know the size of each of Black Eye's sections. But to obtain such knowledge, we would need to compare the galaxy’s apparent size with its known distance. Though M64 certainly contains Cepheid variable stars and brilliant type O stars whose apparent brightnesses should let us measure their distances. But still, experts come up with values that vary more widely than those for other galaxies. 

Astronomer R. Brent Tully’s Nearby lists it at 14 million light-years away. Meanwhile, Robert Burnham describes it somewhere between 20 to 25 million light years away,  Swedish astronomer Erik Holmberg estimates the distance at 44 million light years, while according to a 2004 Hubble Space Telescope press release it lies 17 million light years from us. 

The Black Eye galaxy is speeding away from the milky way at the rate of it’s rushing away from us 234 miles per second which corresponds with a “Hubble flow” distance of 16 million light years, but recessional speeds among galaxies in the Virgo Coma galaxy cluster are often wildly uncorrelated with distance due to the gravitational pull between them. Without knowing its distance, we simply can’t know Black Eye Galaxy's exact dimension. 

If we assume a distance according to what Hubble suggests then it is half the size of our Milky Way Galaxy. But with all that colliding gas and counter rotating segments which results in frenzied star formation, it seems to be a very active galaxy.