Hoag's Object: Evidence for Cold Accretion onto an Elliptical Galaxy
  (Abridged) 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 controversial discussion. 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 nonlinear mechanisms in forming the outer ring was not yet completely
ruled out. These new data, together with HI 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
HI 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. We propose a new scenario where the elliptical
core formed in the early Universe with the HI 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.
