Hoag's Object: evidence for cold accretion on to an elliptical galaxy

We present new photometric and spectroscopic observations of the famous Hoag's Object, a peculiar ring galaxy with a central roundish core. The nature of Hoag's Object is still under controversy. Previous studies demonstrated that a major accretion event that took place at least 2-3 Gyr ago can account for the observational evidence. However, the role of internal non-linear mechanisms in forming the outer ring was not yet completely ruled out.

The observations reported here consist of WFPC2 optical data retrieved from the Hubble Space Telescope archive as well as long-slit and 3D spectroscopic data obtained at the Russian BTA 6-m telescope. These new data, together with H I and optical information from the literature, are used to demonstrate that Hoag's Object is a relatively isolated system surrounded by a luminous quasi-spiral pattern and a massive, low-density H I disc. The main stellar body is an old, mildly triaxial elliptical galaxy with very high angular momentum.

We review previous formation scenarios of Hoag's Object in light of the new data and conclude that the peculiar morphology could not represent a late phase in barred early-type galaxies evolution. In addition, no observational evidence supports late merging events in the evolution of the galaxy, although further tests are required before safely dismissing this idea. Combining all the information, we propose a new scenario where the elliptical core formed in the early Universe with the H I disc forming shortly after the core by prolonged 'cold' accretion of primordial gas from the intergalactic medium.

The low gas density does not allow intense star formation to occur everywhere in the disc, but only along a tightly wound spiral pattern of enhanced density induced by the triaxial gravitational potential. According to this view, the physical mechanism that forms rings in Hoag-like galaxies is closely linked with that in some non-barred disc galaxies, although the formation and evolution of both classes of galaxies are clearly distinct. Whether or not this unique evolutionary track is related to the galaxy residing in an underdensed environment remains to be solved. A detailed H I mapping of Hoag's Object and its environment is required to test our hypothesis and to examine the nature of the H I disc.