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

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.

💡 Research Summary

This paper presents a comprehensive LTE analysis of the molecular gas in the Central Molecular Zone (CMZ) by combining new JCMT 12CO J=2→1 observations with existing CHIMPS2 12CO and 13CO J=3→2 data and SEDIGISM 13CO J=2→1 data. All datasets were convolved to a common 30″ resolution, calibrated to main‑beam temperature, and masked at a 3σ threshold to ensure robust signal detection across the surveyed region (≈358° ≤ ℓ ≤ 1.4°, |b| ≤ 0.5°).

The authors adopt the LTE framework, assuming that the 12CO line is highly optically thick (τ₁₂ ≫ 1) and that a single excitation temperature (T_ex) characterises the gas along each line of sight. Optical depths for 13CO in both the J=3→2 and J=2→1 transitions are derived from the observed brightness‑temperature ratios using τ₁₃ = −ln