The Distribution of Stars Within 2° of The Galactic Centre Revealed by 1 to 2 μm Images

Extract

We have produced images at 1.2, 1.6, and 2.2μm of a 1° × 2° region of the sky that includes the galactic centre using the drift scan techniques described by Catchpole et al. (1985) and Glass et al. (1987). These images were calibrated by observations of standard stars so that it is possible to make quantitative measurements of 3, H and K magnitudes for individual stars. Colour magnitude diagrams have been constructed, using K against H–K, for the stars within elliptical annuli centred on the galactic centre, with major axes along the galactic plane and an axial ratio of 2.2. The distribution of stars within each colour magnitude diagram is compared with a composite giant branch, based on 47 Tuc as seen at a distance of 8.2 Kpc, but extended to higher luminosity by including photometry of long period variables in the Baade windows. The assumption of a unique giant branch for the galactic centre stars is empirically justified by the small scatter in the colour magnitude diagrams beyond about 1.5° from the galactic plane. Fig. 1a & b give examples of colour magnitude diagrams for 400″ × 400″ areas of sky far from and near to the galactic centre. The observed distribution of stars relative to the 47 Tuc giant branch allows us to measure both the foreground absorption and the number distribution of stars within a given range of dereddened apparent magnitude. The number of stars counted must be corrected for the effect of crowding which is estimated by generating simulated star fields and comparing the number of stars found using our reduction programmes, with the number used to simulate the fields.