Hoag's Object: Evidence for Cold Accretion onto an Elliptical Galaxy

TL;DR

This study presents new observations of Hoag's Object, supporting a formation scenario involving early elliptical core formation and subsequent cold accretion of primordial gas, leading to its unique ring structure without recent mergers.

Contribution

It introduces a novel formation model for Hoag's Object involving early universe elliptical core formation and cold gas accretion, challenging previous merger-based explanations.

Findings

Hoag's Object is an isolated system with a luminous quasi-spiral pattern.

The core is an old, mildly triaxial elliptical galaxy with high angular momentum.

No evidence of recent merging events was found.

Abstract

(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.