2013MNRAS.435..475B

We present new Hi observations of Hoag's Object (HO) obtained with the Westerbork Synthesis Radio Telescope. 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 HO, and report on a newly identified SDSS optical companion galaxy. The Hi sources are ∼ 0.3 and ∼ 1Mpc away in projected distance, and the companion galaxy is also ∼ 1Mpc away. Our main conclusion is that the Hi detected in HO shows no indication that this galaxy has experienced a recent (less than ∼ 1Gyr 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 HO, in which case this interaction must also have occurred at least 1-2Gyr ago.