Galaxy population structure from proper motions

Extract

We are currently engaged in astrometry of Kapteyn Selected Area photographic plates covering the period 1908 to the present day using the GALAXY measuring engine at RGO. Our intention is to use the apparent motions to investigate the stellar velocity structure within a kiloparsec of the Sun. We have completed a preliminary analysis of SA 68 (ℓ = 1110, b = 460), deriving positions and proper motions using the central overlap method on measures of plates spanning the period 1909 to 1980. These include the original (1909) and second epoch (1925) plates taken with the Radcliffe refractor at Oxford, and used in the compilation of the Radcliffe Catalogue (Knox-Shaw and Scott-Barrett, 1934), as well as more recent plates taken using the 26-inch refractor at Herstmonceux. For stars present on all thirteen exposures (including double exposures on the same plate) the internal residuals in positions in each co-ordinate are per plate, giving annual motions accurate to ~1 milli-arcsecond over the 71 year baseline. Since our plates have a limiting magnitude of only B ~15, we have used 17 stars in common with Chiu (1980) to transform our relative motions to the absolute frame. Our motions are in very good agreement with those by Chiu, with a scatter of less than 3 milliarcseconds in each co-ordinate and linearity of scale between the two datasets.