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D.E. Hogg, M.S. Roberts (NRAO)
A recent study of one hundred nearby isolated spiral galaxies found that half of the galaxies showed significant asymmetry in their global HI profiles (Haynes et al. AJ,115,62,1998), similar to the proportion found in earlier, more heterogeneous studies. In order to identify the HI responsible for the asymmetry we have used the VLA to map the distribution of neutral hydrogen in three of the galaxies.
In the first case, UGC 3384, the asymmetry is caused by the contribution arising from the emission in a nearby (65 kpc) but previously unrecognized companion. The two dwarf galaxies have similar HI content, and the distribution gives no indication that the galaxies are interacting.
The second example, NGC 1012, is also shown to have a previously unrecognized companion. The companion lies at a projected separation of 55 kpc and is interacting with NGC 1012, witnessed by the distortion of the hydrogen envelope of each object. The optical and HI properties of NGC 1012 are not unusual for an S0/Sa galaxies but the dwarf companion has an unusually high ratio of gas mass to blue luminosity.
The last galaxy, NGC 6239, has the most complex profile, and the neutral hydrogen distribution is severely distorted. A bridge of limited velocity range stretches toward a nearby companion. The material in NGC 6239 extends far beyond the optical disk, and appears to be concentrated into numerous clumps. In view of the HI morphology it is likely that NGC 6239 has experienced a strong tidal interaction recently. However, the radio continuum and far infrared luminosities are both only moderate, indicating an absence of vigorous star formation.
This work is the first stage in a study of HI profile asymmetry. As additional HI maps become available it will be possible to deduce the fraction of nearby apparently isolated spirals which have HI-rich companions, and of these, how many have recently interacted with these companions.
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