`Shell' galaxies typically are isolated, apparently normal elliptical galaxies. On closer examination, they reveal low contrast but quite abrupt changes in their optical luminosity profiles. Some have faint but distinctive, edge-brightened external arcs for which galaxies of this type are named. They are widely considered to be merger remnants, a view reinforced by the discovery that several shell galaxies contain or are associated with significant (i.e. detectable) amounts of neutral hydrogen, and some contain dust. However, neither dust nor gas are regarded as significant components of `normal' elliptical galaxies, but deep optical imaging techniques show that many apparently normal ellipticals, which seem free from gas, dust and shells have unexpected asymmetric low surface brightness features suggestive of interactions, supporting the view that many, perhaps most elliptical galaxies are the result of multiple mergers. This view is reinforced by HI and optical evidence of ongoing merger activity in nearby galaxies.
Deep imaging is increasingly the domain of array detectors, but conventional photography still has an important role, especially in the detection of faint objects of large angular size. This is partly because the excellent uniformity of the photographic detector enables contrast enhancement of plates taken on dark sites to reveal very small signals against the uniform contribution from the sky (Malin 1978). In addition, all of the sky has now been recorded on fine-grain photographic emulsions that lend themselves to such processing, though not all the original plates, which are needed for this process are readily accessible.
Fortunately, this is not the case in the southern hemisphere, and some parts of the southern sky have been recorded on many such exposures, mostly by the UK Schmidt Telescope of the Anglo-Australian Observatory. The plate archive at the Royal Observatory, Edinburgh is a valuable source of such image data. Most of the results reported here derive from UK Schmidt Telescope (UKST) material, and most of it is the result of photographically combining several deep, enhanced images of the same field. Details of this process are given in Malin (1987). The images presented here are components of a photographic atlas of faint features of bright galaxies, currently in preparation.
In this paper, we will present several examples of galaxies where unusual, low surface brightness features are revealed which are strongly suggestive of merger activity and several where these features are correlated with an HI detection. We will also present images of some other galaxy types where anomalous low surface brightness features are strongly suggestive of merger activity but where no HI data exist, a situation likely to be remedied by the proposed ATNF HI survey.