A Megaprime Survey for Structure in the Halo of the Galaxy G.G. Fahlman: CFHT The galactic halo is not a dynamically relaxed system. If the MW has been built through an accretion process, the halo should still bear the signatures of these events. The tidal interactions between satellite stellar systems, dwarf galaxies and globular clusters, have been shown through numerical simulations to leave extended tails of debris strung out along the satellite orbits. These structures are, in some cases, expected to be detectable. In the case of saltellite galaxies the discovery and delineation of these tails would provide direct proof of the hierarchical construction of the MW, a basic tenet of our current theories of galaxy formation and evolution. This is a fairly hard problem to attack, however, because the satellite systems (except in a rare cases - like the Sagittarius Dwarf Galaxy) have melted into the halo and the surviving tidal tails are of low enough contrast that they are not readily detectable. Hence a statistical approach must be followed to discover these tails. One strategy for doing this, Great Circle Cell Counts has been described by Johnson et al (AIP 92, p483, 1995). The method appears to be fairly sensitive but also appears to require a rather large survey area. At this point, I cannot estimate with any confidence the minimum area needed to have a reasonable chance for detecting a tidal stream from a past accretion event but work will continue. The putative survey would require only broad band filters. A niave estimate is that a relatively shallow (V=24) survey of about 400 sq deg would be sufficient to detect a tidal stream if only 10% of the halo were composed of detectable streams). This would take about 100 hours to complete. It should be possible to utilize fields taken in other programs to augment the survey area proposed for this study. Note that the known dSph galaxies appear to be too small to leave tidal streams that would have sufficient contrast to stand out against the average halo surface brightness. In the case of globular clusters, the delineation of the tidal boundary, its shape and structure, will provide important constraints on both the dynamical evolution of these systems and the overall graviational field of the Milky Way. The study of this phenomenon has been pioneered by Carl Grillmair using photographic surveys and more recently he and others have been using CCD cameras for this kind of work. Evidence of tidal tails, some more convincing then others, has been found around many of the galactic globular clusters. All the work to date has been relatively shallow and/or of limited angular extent. Megaprime overs the opportunity to significantly improve upon the current situation. Briefly, the proposed survey would be aimed at obtaining star counts to about V=26 in 4x4 deg fields surrounding all the relatively nearby Northern galactic globular clusters (there are about a dozen suitable candidates). Two colours are needed to clearly separate the cluster stars (bound and unbound) from the general field. Consequently each cluster would require about 32h on the sky. Ideally, some blank fields should be taken for statistical control on background/foreground halo structures. A reasonably complete survey (6 clusters and 3 blank fields) would require about 300 hours in total. As most of the prime targets are at high galactic latitude, the blank fields and the outer fields around the clusters could be used for other science projects. It would be quite worthwhile to do three colour photometry in the proposed blank fields to construct an accurate luminosity function for the halo and thick disk components of the galaxy (three colours are needed to obtain distance estimates). The fields could also be used for weak lensing studies, galaxy morhpology studies, etc.