Distance to three molecular clouds in the central molecular zone

Abstract

Context. The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial for inferring its orbital structure and dynamics and for understanding star formation in the clouds. Aims. Recent work has suggested that the G0.253+0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to crosscheck those results, computing the absolute and relative distances to the Brick as well as to another two molecular clouds (the 50 km s−1 and the 20 km s−1 clouds), and discuss their CMZ membership. Methods. We used the colour magnitude diagrams Ks versus H − Ks to compare stars detected towards the target clouds with stars detected towards three reference regions in the nuclear stellar disc (NSD) and the Galactic bulge. We used red clump (RC) stars to estimate the distance to each region. Results. We found that all the clouds present a double RC feature. Such a double RC has been reported in previous work for the NSD, but not for the bulge adjacent to it. We exclude the possibility that the different RC features are located at significantly different distances. The obtained absolute and relative distances are compatible with the Galactic centre distance (∼8 kpc).