CHIMPS2: The physical properties and star formation efficiency of molecular gas in the Central Molecular Zone

Abstract: We present Local Thermodynamic Equilibrium (LTE) estimates of the physical properties and star formation efficiency (SFE) of molecular gas in the Central Molecular Zone (CMZ), using new $^{12}$CO $J=2\to1$ observations from the James Clerk Maxwell Telescope. Combined with CHIMPS2 $^{12}$CO and $^{13}$CO $J=3\to2$, and SEDIGISM $^{13}$CO $J=2\to1$ data, 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-$μm$ 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 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 be enter a more active phase of star formation, with large reservoirs of gas primed for future activity.