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

ABSTRACT 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 $J=2\rightarrow 1$ observations from the James Clerk Maxwell Telescope. Combined with $^{12}$CO and $^{13}$CO $J=3\rightarrow 2$ data from the CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2), and $^{13}$CO $J=2\rightarrow 1$ data from the Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium (SEDIGISM), we estimate a median excitation temperature of $T_{\rm ex} = 11$ K for $^{13}$CO throughout the CMZ, with peaks exceeding 120 K in the Sgr B1/B2 complex. Cooler gas dominates around Sgr A and nearby clouds. We derive a median H$_{2}$ column density of $N(\mathrm{H}2) = 2 \times 10^{22}$ cm$^{-2}$ and a total $^{13}$CO-traced gas mass of $M_{\rm gas} = 7 \times 10^6$ M$_\odot$, consistent with previous estimates when accounting for spatial coverage. The instantaneous SFE is assessed using Hi-GAL compact sources detected at 70 $\rm{\mu m}$ and 160–500 $\rm{\mu m}$. The 70-$\rm{\mu m}$-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 $\rm{\mu m}$ SFE, tracing cold pre-stellar gas, is more 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 protostars. Our results imply that the CMZ may enter a more active phase of star formation, with large reservoirs of gas primed for future activity.