STELLAR POPULATIONS AND LARGE-SCALE STRUCTURE OF THE SMC .3. THE GEOMETRY OF THE NORTHERN AND NORTH-WESTERN OUTLYING REGIONS

The third paper in the series of investigations on the large-scale structure of the Small Magellanic Cloud is presented here. Following the study by Hatzidimitriou & Hawkins of a large area covering nearly 50 square degrees to the north-east and southwest of the SMC main body, further results from the northern and north-western areas have been obtained. Photographic photometry derived from COSMOS analysis of sets of blue and red UK Schmidt plates provided the data for the study. Results on the projected two-dimensional appearance of the SMC and the three-dimensional geometry of these areas are presented on the basis of the properties of the red horizontal branch/clump stellar population. It was found that the SMC outer regions have a greater projected extent from the SMC centre in the northern direction compared to the western direction, the distortion probably resulting from interaction with the tidal field of the Galaxy. The structure of the clump of red horizontal branch stars on an areal grid of R versus B-R colour-magnitude diagrams enabled the examination of the distance and depth structure of the studied regions. The area of large depth along the line-of-sight direction, reported in the NE field in the second paper of the series, has been found to extend about one degree further westward into the adjacent field with values for the depth (here defined as the range of distances within which 68.3 per cent of the stars lie) between 12 and 16 kpc. However, most of the area in the north and north-western outer regions exhibits a small depth of between 4 and 9 kpc. The data indicate the existence of a distinct structure in the north-east lying projected in front of the main structure of the SMC. Its origins are thought to be due to the close encounter between the LMC and SMC 2 x 10(8) yr ago suggested by the tidal models of Fujimoto & Murai.