The large asymmetric HI envelope of the isolated galaxy NGC 864 (CIG 96)

¹ Instituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18080 Granada, Spain e-mail: daniel@iaa.es
² Observatoire de Marseille, 2 Place le Verrier, 13248 Marseille Cedex 4, France
³ Department of Astronomy, University of Alabama, AL 35487, USA
⁴ Department of Astronomy, University of Massachussetts, Amherst, MA 01003, USA

Received: 21 January 2005
Accepted: 23 June 2005

Abstract

We present an HI synthesis imaging study of NGC 864 (CIG 96), a spiral galaxy well isolated from similarly sized companions, yet presenting an intriguing asymmetry in its integral HI spectrum. The asymmetry in the HI profile is associated with a strong kinematical perturbation in the gaseous envelope of the galaxy, where at one side the decay of the rotation curve is faster than Keplerian. We detect a small (M(HI) = 5 10⁶ ) galaxy with a faint optical counterpart at ~80 kpc projected distance from NGC 864. This galaxy is probably not massive enough to have caused the perturbations in NGC 864. We discuss alternatives, such as the accretion of a gaseous companion at a radial velocity lower than the maximum.