Searching for the in-plane Galactic bar and ring in DENIS

Abstract

New evidence for a long thin Galactic bar (in contradistinction to the bulge), as well as for the existence of the ring and the truncation of the inner disc, are sought in the DENIS survey. First, we examine DENIS and Two Micron Galactic Survey star counts for the characteristic signatures of an in-plane bar and ring. The star counts in the plane for 30° > l > -30° are shown to be highly asymmetric with considerably more sources at positive than at negative longitudes. At |b| ≈ 1.5°, however, the counts are nearly symmetric. Therefore, the asymmetry is not due to the disc, which is shown to have an inner truncation, or to the bulge, so there has to be another major component in the inner Galaxy that is causing the asymmetries. This component provides up to 50% of the detected sources in the plane between the bulge and l = 27° or l = -14°. This component is shown to be consistent with an in-plane bar with a position angle of 40° and half-length of 3.9 kpc. However, there is also a major peak in the counts at l = -22°, which coincides with the tangential point of the so-called 3 kpc arm. This is shown to be most probably a ring or a pseudo-ring. The extinction in the plane is also shown to be asymmetric with more extinction at negative than at positive longitudes. For l < 8° the extinction is shown to be slightly tilted with respect to b = 0° in the same manner as the HI disc. We conclude that the Galaxy is a fairly typical ringed barred spiral galaxy.