Detection of the old stellar component of the major Galactic bar

Abstract

We present near-infrared colour-magnitude diagrams and star counts for a number of regions along the Galactic plane. It is shown that along the l = 27°, b = 0° line of sight there is a feature at 5.7 ± 0.7 kpc with a density of stars at least a factor of 2 and probably more than a factor of 5 times that of the disc at the same position. This feature forms a distinct clump on an H versus J - H diagram and is seen at all longitudes from the bulge to about l = 28°, but at no longitude greater than this. The distance to the feature at l = 20° is about 0.5 kpc further than at l = 27°. and by l = 10° it has merged with, or has become, the bulge. Given that at l = 27° and l = 21° there is also a clustering of very young stars, the only component that can reasonably explain what is seen is a bar with half-length of around 4 kpc and a position angle of about 43° ± 7°.