3-D CMZ III: Constraining the 3-D structure of the Central Molecular Zone via molecular line emission and absorption

TL;DR

This study uses molecular line emission and absorption data to analyze the 3-D structure of the Milky Way's Central Molecular Zone, revealing a complex, asymmetric distribution of clouds and limitations of existing orbital models.

Contribution

It provides new constraints on the line-of-sight positions of CMZ clouds and introduces a revised toy model, highlighting the complexity of the 3-D structure beyond simple geometric models.

Findings

Most clouds are in front of the Galactic centre

Significant inconsistencies in near vs. far cloud placement

The CMZ's structure is more complex than simple models

Abstract

The Milky Way's Central Molecular Zone (CMZ) is the largest concentration of dense molecular gas in the Galaxy, the structure of which is shaped by the complex interplay between Galactic-scale dynamics and extreme physical conditions. Understanding the 3-D geometry of this gas is crucial as it determines the locations of star formation and subsequent feedback. We present a catalogue of clouds in the CMZ using Herschel data. Using archival data from the APEX and MOPRA CMZ surveys, we measure averaged kinematic properties of the clouds at 1mm and 3mm. We use archival ATCA data of the H$_{2}$CO (1$_{1,0}$ - 1$_{1,1}$) 4.8 GHz line to search for absorption towards the clouds, and 4.85 GHz GBT C-band data to measure the radio continuum emission. We measure the absorption against the continuum to provide new constraints for the line-of-sight positions of the clouds relative to the Galactic centre, and find a highly asymmetric distribution, with most clouds residing in front of the Galactic centre. The results are compared with different orbital models, and we introduce a revised toy model of a vertically-oscillating closed elliptical orbit. We find that most models describe the PPV structure of the gas reasonably well, but find significant inconsistencies in all cases regarding the near vs. far placement of individual clouds. Our results highlight that the CMZ is likely more complex than can be captured by these simple geometric models, along with the need for new data to provide further constraints on the true 3-D structure of the CMZ.