One galaxy or two?

From W.J.J. Hoge:

Is this one galaxy or two? That question came up back in 1950 when astronomer Art Hoag (a very distant relative) found this unusual extragalactic object. The ring full of bright blue stars surrounds a center of older, redder stars. The gap appears almost completely dark. How Hoag’s Object formed is unknown, but similar objects have now been identified and collectively labeled as a form of ring galaxy. Best guesses include a galaxy collision billions of years ago and the gravitational effect of a central bar that has since vanished. This photo was taken by the Hubble Space Telescope in 2001 and shows unprecedented details of Hoag’s Object. It spans about 100,000 light years and is around 600 million light years away toward the constellation of the Serpens (the Snake). Another ring galaxy is visible in the gap (at about one o’clock).

Image Credit: NASA