NGC 1961: Stripping of a supermassive spiral galaxy.

The peculiar supermassive Sb spiral galaxy NGC 1961 has been mapped in the 21 cm line of neutral hydrogen with a resolution of 0.5 arcmin using the Westerbork Synthesis Radio Telescope. An unusual 'head-tail' morphology for the H I is found, consisting of a wing of gas extending 30 kpc to the NW, and a sharp edge to the SE. The latter lies well within the 26.5 m/sq arcsec blue optical contour (Holmberg radius). Radio continuum emission is observed in a ridge coincident with the SE spiral arm, and an X-ray map produced with the IPC of the Einstein satellite reveals an arc-shaped structure which is also aligned with the ridge. These observations are interpreted as indicating that the gas in NGC 1961 is being stripped by a hot intergalactic medium of density of at least 0.001/cu cm and total mass more than 10 billion solar masses. From the observed morphology, this gas would be located in the direction of the center of mass of a group of galaxies located SE of NGC 1961. Five group members have also been detected in H I.