The Structure and Evolution of Hoag's Object

The authors present new imaging, photometric, and spectroscopic observations of Hoag's object, a 16th magnitude galaxy consisting of an almostly perfectly round core surrounded by a faint, apparently detached ring. The core of this distant system appears to be a normal spheroid with a half-light radius of 3.6 kpc, and an apparent rotation at r = 2.5 kpc of V_rotsin i = 18 km s^-1. The ring is of comparable luminosity, has a mean radius of 23 kpc, is inclined about 19°±5° to the plane of the sky, and shows knotty structure and gaseous emission lines indicative of young stars. The ring rotates with V_max= 300(+100,-60) km s^-1. The authors rule out several earlier hypotheses on the origin of the ring and propose the new hypothesis that Hoag's object owes its structure to a major accretion event at least 2-3 Gyr ago.