Introduction

Many spiral galaxies display clear asymmetries in their light distribution (Rix & Zaritsky 1995; Zaritsky & Rix 1996), including some which are apparently quite isolated and non-interacting. Often, studies of the underlying mass distribution of neutral hydrogen reveal much of this asymmetry to be mainly ``cosmetic'', attributable to (in the case of NGC 1313; Ryder, G. Purcell, D. Davis, V. Andersen et al. 1995) such factors as the presence of an HI superbubble. However, the recognition of an inherently non-circular and asymmetric population of galaxies would have major ramifications for topics as fundamental as the derivation of disk inclinations, dark matter halos, and the Tully-Fisher relation.

 
Figure 1:  B-band CCD image of NGC 7421 taken with the MSSSO 1.0 m telescope. North is up, and east is to the left. Note the ``bow-shock'' morphology of the western side, compared with the extended, low surface brightness envelope ( mag arcsec) to the east.

NGC 7421 is a SB(r)bc galaxy, seen at low inclination, that appears to sit slightly off-centre in a low surface brightness envelope some 2' in diameter (Figure 1). NGC 7421 exhibits a complete inner resonance ring (Buta 1995), but instead of a complete outer ring, there is a semi-circular arc of HII regions forming a sharp western boundary to the galaxy. Since NGC 7421 is a prominent member of Group No. 40 in the Catalog of Southern Groups of Galaxies (Maia et al. 1989), one gets a strong impression that this arc may represent some sort of a ``bow-shock'' effect as NGC 7421 ploughs its way through the intracluster medium, while the off-center envelope may represent the ``wake'' of its passage. Such bow-shock morphologies are also observed in NGC 2276 (Gruendl et al. 1993) and in NGC 4273 (Davis et al. 1995), both of which are members of groups containing moderate amounts of hot intra-group gas (though probably not enough to explain the presence of the bow-shock).

In order to test the hypothesis that NGC 7421 is interacting with an intracluster medium, we are currently analysing some 16000 seconds of publicly-available ROSAT Position Sensitive Proportional Counter (PSPC) observations to check whether the density of diffuse hot gas in this group is sufficient to account for any gas sweeping. The dynamics of the ionised gas in the inner disk of NGC 7421 have been looked at with the Rutgers Fabry-Perot spectrometer on the CTIO 4 m telescope (Purcell & Buta 1997). We have also mapped the distribution and kinematics of the HI gas in NGC 7421 using a total of 5 configurations (0.375, 750C, 1.5D, 6C, and 6D) of the Australia Telescope Compact Array (ATCA) between 1995 September and 1996 January, and we present here some preliminary results from these observations.

© Copyright Astronomical Society of Australia 1997