The nature of Hoag's object : the perfect ringed galaxy.

Hoag's object, a peculiar ringed galaxy, has been observed photographically, photometrically and spectroscopically in the optical part of the spectrum, and in the 21-cm radio band. The new observations, together with previously published data, indicate that the ring may have been formed a few Gyrs ago as a result of an extreme bar instability in a galactic disk. In this case, the galaxy may be lacking the massive halo which, in most instances, stabilizes galaxy disks against this instability.