HI in HO: Hoag's object revisited

TL;DR

This study presents new HI observations of Hoag's Object revealing a large, warped HI structure aligned with its optical ring, and discusses the galaxy's interaction history based on nearby HI sources and a companion galaxy.

Contribution

The paper provides detailed HI mapping of Hoag's Object and identifies additional HI sources and a companion galaxy, offering insights into its interaction history and stability.

Findings

The HI structure is twice as large as the optical ring and mildly warped.

No evidence of recent (<1 Gyr) accretion events in Hoag's Object.

Detected two additional HI sources and a new optical companion galaxy.

Abstract

We present new HI observations of Hoag's Object obtained with the Westerbork Synthesis Radio Telescope (WSRT). The data show that the luminous optical ring around the elliptical body has a bright HI counterpart that shares the kinematical properties of the optical ring. The entire HI structure is twice as large as the optical ring and shows a mild warp in its outer regions relative to the inner ring. We detect two additional HI sources close in redshift to that of Hoag's Object, and report on a newly identified SDSS optical companion galaxy. The HI sources are ~0.3 and ~1 Mpc away in projected distance, and the companion galaxy is also ~1 Mpc away. Our main conclusion is that the HI detected in Hoag's Object shows no indication that this galaxy has experienced a recent (less than ~1 Gyr ago) accretion event. At least one of the two additional HI detected objects does not have an optical counterpart. One possibility is that this object is an HI filament left over from an interaction shaping Hoag's Object, in which case this interaction must also have occurred at least 1-2 Gyr ago.