CHIMPS2: the physical properties and star formation efficiency of molecular gas in the central molecular zone

We present Local Thermodynamic Equilibrium estimates of the physical properties and star formation efficiency (SFE) of molecular gas in the central molecular zone (CMZ), using new ¹² CO J = 2 → 1 observations from the James Clerk Maxwell Telescope. Combined with ¹² CO and ¹³ CO J = 3 → 2 data from the CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2), and ¹³ CO J = 2 → 1 data fr om the Structur e, Ex citation and Dynamics of the Inner Galactic Int erst ellar Medium (SEDIGISM), we estimate a median e x citation temperatur e of T _ex = 11 K for ¹³ CO throughout the CMZ, with peaks e x ceeding 120 K in the Sgr B1/B2 comple x. Cooler g as dominates ar ound Sgr A and nearby clouds. We derive a median H ₂ column density of N(H2) = 2 ×10 ²² cm⁻² and a total ¹³ CO-traced gas mass of M_gas = 7 ×10⁶ M_⊙ , consistent with previous estimates when accounting for spatial coverage. The instantaneous SFE is assessed using Hi-GAL compact sources det ect ed at 70 μm and 160–500 μm . The 70- μm -bright SFE, tracing current star formation, is modest overall but elevated in Sgr B1/B2, the Arches cluster, and Sgr C. In contrast, the 160–500 μm SFE, tracing cold pr e-stellar g as, is mor e broadly enhanced, particularly in the dust ridge clouds and towards negative longitudes surrounding Sgr C. The contrasting distributions suggest an evolutionary gradient in SFE, consistent with a transition from dense, cold gas to embedded pr otostars. Our r esults imply that the CMZ may enter a mor e active phase of star formation, with large r eservoirs of g as primed for future activity.