ATCA Results

 
Figure 2:  Contours of HI column density (from 5 configurations of the ATCA) overlaid on a B image of NGC 7421 (from the Digitised Sky Survey). The contours correspond to (projected) column densities of 0.2, 0.4, 0.9, 1.8, 2.6, 3.5, 4.4, 5.3, and  cm.

Figure 2 shows contours of the HI column density (resolution ) overlaid on an optical image of NGC 7421 from the Digitised Sky Survey. The gas disk extends north-south (36 kpc at a distance of 17.8 Mpc; Tully 1988), over 3 times the size of the optical envelope. The impression of a ``bow-shock'' from the optical disk is reinforced by the shape of the HI disk. The northern and southern cusps are consistent with the galaxy ploughing (or rather ``surfing'', since the gas has not been entirely stripped) its way westwards through some intracluster medium. There is not significantly more gas underlying the western arc of HII regions than in the eastern half, though vigorous star formation on the western side may have already consumed much of the accumulated gas.

 
Figure 3:  Velocity field of NGC 7421 overlaid on a grey-scale of the HI column density. The velocity contours span the range from 1730 km s (northeast) to 1840 km s (western edge of disk).

Figure 3 shows iso-velocity contours from the velocity field overlaid on a grey-scale of the HI column density shown in Figure 2. At first glance, the inner velocity field looks fairly normal for a rotating galaxy disk, but there are two highly unusual aspects to it. First, the iso-velocity contours close up on themselves about the kinematic minor axis, but even more intriguing is that the kinematic line-of-nodes (the normal to the iso-velocity contours), and the major axis of the HI isophotes, are perpendicular to each other, instead of the conventional parallel alignment. It is possible that ram pressure has ``squeezed'' the disk, but then rotation ought to result in elongation of the disk preferentially in one direction. Alternatively, the gas may actually be ``tumbling'' about the galaxy's major axis, as in the amorphous dwarf starburst galaxy NGC 5253 (Kobulnicky & Skillman 1995).

The global HI profile obtained by integrating the emission within each channel map yields a flux integral of 20.2 Jy km s, which equates to a total HI mass of  M. The profile width at 20% of the peak flux (uncorrected for inclination) is only 147 km s. Neutral hydrogen in the ``wake'' has been swept to lower velocities, giving rise to a ``single-horn'' profile. Such asymmetries in single-dish spectra may be the signature of galaxies akin to NGC 7421 (Richter & Sancisi 1994).

We were able to successfully extract HI data for NGC 7418 (another member of the same group as NGC 7421) from the same observations, even though it lies just outside the 34' primary beam of the ATCA. The distortion of the gas isophotes in NGC 7418 is consistent with the more usual type of warp, but is further evidence of the turbulent intracluster environment in which NGC 7421 and NGC 7418 find themselves.

© Copyright Astronomical Society of Australia 1997