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1 Introduction

The extensive $\approx 6.2\hbox{$^\circ$}\times 6.2\hbox{$^\circ$}$ observing field of the United Kingdom 1.2-m Schmidt Telescope (UKST) of the Anglo-Australian Observatory (AAO), allows the whole (or significant fractions) of nearby clusters of galaxies to be imaged simultaneously. The most sensitive photographic emulsions are required for such observations as most regions of extragalactic objects are of very low surface brightness, and techniques such as photographic amplification or co-addition (Malin 1979) increased the depth to which such faint features of galaxies could be identified on Schmidt plates.

At the end of the 1980s, increased computing capability and disk storage facilities allowed for the possibility of digitally co-adding microdensitometer scans of whole Schmidt plates in order to increase the signal-to-noise ratio (S/N) and enable the detection of fainter extended features of galaxies, and produce a resultant co-added data array from which fully quantitative results could be obtained. As a preliminary project, Kemp & Meaburn (1991a) co-added Automatic Plate Measuring (APM) machine scans of 8 IIIaJ sky-limited plates of a field containing the IC 4296 cluster of galaxies, for the purpose of detecting faint haloes around galaxies and other extensive low surface brightness features; the scanned area was about $4.5\hbox{$^\circ$}\times
4.5\hbox{$^\circ$}$. Discoveries were reported in Kemp & Meaburn (1991a,b, 1993, 1994, 1995) and Kemp (1994) and included: a giant halo of dimensions $\sim 600 \times 200$ kpc around a cD galaxy; tidal tails, detached filaments and distortions associated with a pair of interacting galaxies; a warped disk of an edge-on spiral galaxy; a set of five spectacular trails "emanating" from an apparently normal-looking lenticular galaxy; a possible "shell" feature associated with a late-type spiral galaxy; faint optical emission spatially associated with the radio lobes of IC 4296; and a halo surrounding an irregular SMC-like galaxy which is almost perfectly circular in projection. The faintest features were estimated to be at approximately 27 mag arcsec-2 in the $B_{\rm J}$ band, while surface brightness profiles could be followed to about 1 mag arcsec-2 below this. A full photometric calibration of this data is now being carried out using Anglo-Australian Telescope (AAT) and South African Astronomical Observatory charge-coupled-device (CCD) data.

Recently, the availability of hyper-sensitised Kodak Technical Pan (TechPan) films (Kodak 1981; Russell et al. 1992) at the UKST has increased the possibilities for projects studying faint features of galaxies, at least for the red end of the spectrum. Phillipps & Parker (1993) carried out tests on two films during the initial trials of TechPan film at the UKST. These used the same OG590 filter and plateholder combination employed for normal "OR" R-band IIIaF exposures. The difference in quality between the films and the IIIaF plates was easily noticeable visually, the galaxy images on the TechPan films having much greater clarity, due to the Tech-Pan emulsion having extremely fine grain size, low rms diffuse granularity and high resolving power (Phillipps & Parker 1993 and references within). In good seeing, sky-limited exposures with TechPan films are reported to go a magnitude or more deeper than those with the equivalent IIIaF plates. They found a limiting isophote for galaxy surface photometry of $\sim\! 27$ mag arcsec-2 (0.2% of the sky background) for these films.

The RCA CCD combined with the Anglo-Australian Telescope (3.9 m aperture) would take only 2 minutes to achieve the same S/N (i.e. pre unit area for low surface brightness sources) as obtained in the individual films. However, the greater sensitivity of the CCD, for any project which involves observing extended objects or fields, is more than offset by the cumulative exposure involved in mosaicing a large number of fields. Modern CCDs, on 4 m-class telescopes, typically cover fields of the order of 0.01 square degrees compared with 40 square degrees for UKST plates. In addition, the mosaicing of CCD fields creates problems with matching discontinuities between adjacent fields. Of course, the two types of detector are complementary, in that objects identified as interesting in the co-added array can be targeted for multicolour CCD follow-up observations, which can give information on the stellar populations contained in them etc. and we will be reporting such work in the future.

After the success of the initial digital plate co-addition project of Kemp & Meaburn (1991a), the Virgo cluster became the natural follow-up target, as it is the nearest rich cluster of galaxies and has a high galactic latitude which minimises foreground galactic contamination. Thirteen sky-limited TechPan R-band films of the Virgo cluster were used. The $\approx\!
 6.2\hbox{$^\circ$}\times 6.2\hbox{$^\circ$}$ area of each film was digitally scanned and the resultant arrays co-added as described below, to produce a digitally co-added array of unprecedented area. New software had to be developed to cope with the alignment and co-addition of such large data files, and for making a non-symmetric vignetting correction to the stacked image. With the co-addition of 13 films galaxy surface photometry is expected to be possible to below 28 R mag arcsec-2 (0.1% of sky) over large angular scales. Initial priorities for investigation included: study of warping in the 20 or so edge-on disk galaxies contained in the area; giant haloes around elliptical galaxies, including M 87; comparison of the low surface brightness intra-cluster medium with maps of the cluster at other wavelengths e.g. X-ray and radio; and a general study of the amount of baryonic dark matter which is visible at very low surface brightnesses. In this paper we assume all Virgo cluster members are at a distance of 17 Mpc (Mould et al. 1995), resulting in a spatial scale of 5 kpc arcmin-1.

This paper reports the software developed and utilised to produce the co-added array of unprecedented spatial area. Some preliminary results are then displayed and described for a number of galaxies in the field. In some cases e.g. M 87 and M 89 well-known galaxies and previously discovered features are seen with the new clarity of TechPan film and new depth produced by the co-addition. In other cases e.g. NGC 4435/4438 and IC 3481/3481A new low surface brightness features indicating interactions between galaxies or disturbances in the outer parts of galaxies have been discovered. Further papers will report these and follow-up CCD observations in more detail.

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