The structure and evolution of Hoag's object

Abstract

We 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 (cz0 = 12,735 km s_1) appears to be a normal spheroid for its luminosity (MB = — 20.43 for H0 = 50 km s_1 Mpc-1), with a well-defined r1/4 law, a half-light radius re = 3.6 kpc, a central velocity dispersion av= 154 + 6 km s_1, and an apparent rotation at r = 2.5 kpc of Vroi sin i = 18 km s_1. The ring is of comparable luminosity (MB = —20.55), has a mean radius of 18'.'5 = 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 two-horned 21 cm line profile measured at Arecibo shows that the ring rotates with Vmax — 300^60° km s"1 and has an H i mass normal for the system’s luminosity, MHI = 1.8(±0.2) x 1010 M0. The absolute magnitude of the galaxy is MB = —21.24, and the blue mass-to-light ratio within r= 32 kpc is M/LB — 14^5° (M/Lb)q. Our observations rule out previous hypotheses that the ring may be (1) the gravitationally lensed image of a background galaxy or (2) the ring-wave response of a galactic disk to the central passage of a companion, and provide evidence against the recently advanced hypothesis that (3) the ring formed in response to an extreme bar instability in a galactic disk. Instead, we propose the new hypothesis that Hoag’s object owes its structure to a major accretion event at least 2-3 Gyr ago. We point out a number of similar galaxies