3-D CMZ II: Hierarchical Structure Analysis of the Central Molecular Zone

TL;DR

This paper investigates the 3-D hierarchical structure of the Central Molecular Zone in the Milky Way, analyzing physical and kinematic properties of structures, their star formation activity, and pressure conditions, revealing insights into its complex gas dynamics.

Contribution

It provides a detailed hierarchical analysis of the CMZ's structures, comparing their properties with other galactic regions and exploring their pressure and star formation characteristics.

Findings

CMZ structures generally follow star-formation relations.

Most structures are pressure-bounded and in virial equilibrium.

Column density PDFs have steeper power-law slopes than previously reported.

Abstract

The Central Molecular Zone (CMZ) is the way station at the heart of our Milky Way Galaxy, connecting gas flowing in from Galactic scales with the central nucleus. Key open questions remain about its 3-D structure, star formation properties, and role in regulating this gas inflow. In this work, we identify a hierarchy of discrete structures in the CMZ using column density maps from Paper I (Battersby et al., submitted). We calculate the physical ($N$(H$_2$), $T_{\rm{dust}}$, mass, radius) and kinematic (HNCO, HCN, and HC$_3$N moments) properties of each structure as well as their bolometric luminosities and star formation rates (SFRs). We compare these properties with regions in the Milky Way disk and external galaxies. Despite the fact that the CMZ overall is well below the Gao-Solomon dense gas star-formation relation (and in modest agreement with the Schmidt-Kennicutt relation), individual structures on the scale of molecular clouds generally follow these star-formation relations and agree well with other Milky Way and extragalactic regions. We find that individual CMZ structures require a large external pressure ($P_e$/k$_B$ $> 10^{7-9}$ K cm$^{-3}$) to be considered bound, however simple estimates suggest that most CMZ molecular-cloud-sized structures are consistent with being in pressure-bounded virial equilibriuim. We perform power-law fits to the column density probability distribution functions (N-PDFs) of the inner 100 pc, SgrB2, and the outer 100 pc of the CMZ as well as several individual molecular cloud structures and find generally steeper power-law slopes ($-9<\alpha<-2$) compared with the literature ($-6 < \alpha < -1$).