Distance to three molecular clouds in the central molecular zone

TL;DR

This study determines the distances to three molecular clouds in the central molecular zone using stellar color-magnitude diagrams, confirming their association with the Galactic center and clarifying their orbital positions.

Contribution

The paper provides new distance estimates for three molecular clouds in the CMZ, challenging previous claims about the Brick's non-membership, and refines understanding of their placement within the Galactic center.

Findings

All three clouds are consistent with being in the CMZ near the Galactic center.

The double red clump feature indicates similar distances for the clouds and the nuclear stellar disc.

The results support the clouds' membership in the CMZ at about 8 kpc from Earth.

Abstract

The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial to infer its orbital structure, dynamics, and to understand star formation in the clouds. Recent results by Zoccali et al. 2021 suggest that the G0.253 + 0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to cross check their results computing the absolute and relative distance to the Brick and also to other two molecular clouds (the 50 km/s, and the 20 km/s clouds), and discuss their CMZ membership. We used the colour magnitude diagrams $K_s$ vs. $H-K_s$ 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. We obtained 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 ($\sim8$ kpc).