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

TL;DR

This work uses LTE analyses of multiple CO transitions to map the physical state of CMZ molecular gas, deriving $T_{\rm ex}$, $N(^{13}{\rm CO})$, and $M_{\rm gas}$ across the region. It combines CHIMPS2 and SEDIGISM data with supplementary JCMT observations, enabling per-pixel LTE inferences and a robust mass estimate of $M_{\rm gas} \approx 7\times10^{6}$ M$_{\odot}$ within the studied area. By correlating Hi-GAL infrared luminosities with gas mass, the authors construct two SFE maps: a 70 $\mu$m-bright map showing current star formation concentrated in a few regions, and a 160–500 $\mu$m map indicating broad incipient star formation potential, implying an evolutionary gradient and a potential rise in CMZ activity. Collectively, the results provide LTE-based gas properties and spatially resolved SFE that inform models of gas inflow, cloud evolution, and star formation in the Galactic Centre's extreme environment.

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.

Figures (11)

• Figure 1: The peak main-beam temperature maps of the CMZ before smoothing for the transitions and isotopologues of CO. Top: $^{12}$CO $J = 2\to1$ (this work). The masked pixels (NaN values) are those with SNR $< 5$.Second: $^{13}$CO $J = 2\to1$Schuller2017. Third: $^{12}$CO $J = 3\to2$Eden2020. Bottom: $^{13}$CO $J = 3\to2$King2024. The top, third, and bottom maps were smoothed for the analysis to match the resolution of the second panel. The white dashed lines divide the regions containing one of the Sagittarius complexes (Sgr B, Sgr A, and Sgr C from left to right.)
• Figure 2: The $J=3\to2/J=2\to1$ optical-depth ratio of $^{13}$CO as a function of excitation temperature ($T_{\rm ex}$) of $^{13}$CO determined using Equation \ref{['Eqn: brents']}. The red line represents the high-temperature limit at $\tau_r = 2.25$, above which no solution is found.
• Figure 3: Histogram of the distribution of $^{13}$CO excitation temperature ($T_{\rm ex}$) values, calculated from the data under the assumption of Local Thermodynamic Equilibrium (LTE). The median $T_{\rm ex}$ is $11^{+2}_{-2}$ K. The excitation temperature values were determined using the method described in Section \ref{['subsec: Temperature']}. The inset plot shows the same distribution on a log-$y$ scale.
• Figure 4: The distribution of the excitation temperature ($T_{\rm ex}$) across the Central Molecular Zone (CMZ), derived as in Section \ref{['subsec: Temperature']}. The distribution reaches its peak towards Sgr B, and is similar to the Hi-GAL dust temperature map shown in Molinari2011. The contour shows the $^{12}$CO $J = 3 \to 2$ peak intensity from Fig. \ref{['fig:13CO_comparison']} at 30 K (black), 60 K (green) and 90 K (purple). Grey pixels represent regions where $\tau_r > 2.25$, thus no valid solution to Equation \ref{['Eqn: brents']} is found.
• Figure 5: The H$_{2}$ column density across the CMZ. The peak column density of $2 \times 10^{25}\mathrm{cm}^{-2}$ is observed towards the Sgr B1/B2 complex (G0.070-0.059). The contour shows the $^{12}$CO $J = 3 \to 2$ peak intensity at $T_{\rm mb} = 30$ K (white), $60$ K(dark-grey), and $90$ K(red).