4 Conclusion

This paper illustrates preliminary results obtained from the digital co-addition of 13 APM scans of Kodak TechPan UKST films of a field containing the SE part of the Virgo cluster. With this technique we can detect previously unseen low surface brightness features of galaxies over the whole of the $\approx\! 6.2\hbox{$^\circ$}\times 6.2\hbox{$^\circ$}$ area scanned, producing data with an increased dynamic range and which can be calibrated photometrically using CCD observations of objects within the field. Hence we can make full quantitative use of this data array, and this important calibration will be reported in a future paper (some preliminary calibration results are given in this paper). Also with such digital data, processing techniques such as removal of stellar images can be performed, while methods such as "unsharp masking" are greatly facilitated. Mosaicing of CCD frames is not yet competitive in terms of covering such large fields in equivalent observing and processing times, and has problems with matching backgrounds at the edges of frames. Many of the galaxies reported here would require mosaicing of large numbers of contiguous CCD fields to cover their apparent area.

While processing the film scans we noted that there were variations in emulsion sensitivity over the wide field of the films, varying in position from film to film. In the co-added image it appears that the faintest structures we detect are reliable up to scales of about 1 square degree on average. In Fig. 6 we see a strong suggestion of very faint intra-cluster medium emission occupying the northern part of the field (central area of the cluster), but the reality of these structures needs to be tested with accurate calibration of each film, further films and independent measurements. This is one of our priorities for future work, as is investigating the causes of the apparent variation in emulsion sensitivity across each film.

Nevertheless the TechPan films clearly provide good imaging quality, even with the slight blurring introduced by digital aligning and co-addition (errors in alignment are only a small fraction of a pixel), while new faint features of galaxies have been discovered, even though many of these galaxies have been studied before (some previous studies have been at blue wavebands, where the relatively faint sky level makes low surface brightness structures easier to detect). When the data array is satisfactorily calibrated, we will present the accurate surface brightnesses for the features detected here and the gain in magnitude obtained by the co-addition.

With a single film features should be detected at surface brightnesses as low as 27 R mag arcsec^-2 (Phillipps & Parker 1993). As the co-addition of 13 films should reduce the sky photon noise per pixel by a factor $\sqrt{\frac{2\times13}{\pi}}$ ($\approx\! 2.9$), then features as low as 28.1 R mag arcsec^-2 should be visible in the co-added array. The CCD data from the sky area around the galaxies IC 3481 & IC 3481A were used to obtain a preliminary photometric calibration and a sky noise level of 27.9 $\pm$ 0.3 mag arcsec^-2 was measured for the co-added array for the area around these galaxies.

The problems associated with the processing of large (222 Mb) data files have been overcome by a combination of modifying existing Starlink software, and writing new software. These software items have been used to provide new ways of implementing techniques such as alignment, normalisation, co-addition, correction for vignetting effects, and removal of stellar features. Positional information was also stored in the final co-added array.

Some new astrophysical results are presented here. Filaments are seen associated with the apparently interacting galaxies NGC 4435 and NGC 4438. Connecting filaments are seen between the galaxies IC 3481 and IC 3481A for the first time, and a further "loop" of filaments apparently connecting these two galaxies with IC 3483 (though we note the discrepant redshift of the latter). Also more high-quality images are presented of previously detected features such as the N filament associated with NGC 4435 and NGC 4438, and the shells and "jet" features of M 89. Also deep images are presented of the giant haloes surrounding the ellipticals M 87, M 84 and M 86, although, perhaps surprisingly, no trace of distortions are found in the halo shapes of galaxies apparently adjacent to them, some of which might have been thought to be interacting with them. Co-addition of further films will clarify various uncertain features (e.g. whether or not there are any more shells in the vicinity of M 89 or further filaments associated with NGC 4435/8).

Complete CCD calibration and multicolour photometry will be important for the scientific interpretation of these features. We will also estimate the mass of luminous material contained within the haloes of galaxies in the field. Comparison of the revealed halo sizes with the extent of X-ray emission detected by ROSAT and other X-ray satellites show that there is no consistent correlation between the R-band and X-ray haloes. The X-ray haloes can be much larger than the optical haloes (as in the case of M 87) or much smaller, nevertheless the combination of visible ("red-emitting") mass in the haloes detected here and the X-ray-emitting mass can provide new estimates of the total luminous baryonic masses of galaxies.

Acknowledgements

We are particularly grateful to the staff of both the UKSTU and the ROE for the provision of the photographic material, their help on various occasions and the useful discussions. We also thank the Royal Greenwich Observatory (RGO) and specially Dr. M. Irwin for the scanning of the films. Many thanks also to Dr. B. Kellett for his hospitality and the interesting discussions, and Dr. R.J.R. Williams, Dr. Q.A. Parker, Dr. S. Phillipps and J.-M. Schwartzenberg for further interesting and useful discussions.