The Galactic Centre G+0.633-0.0604 molecular cloud: a new astrochemical gold mine. I. Gas physical properties

Abstract: In the Central Molecular Zone (CMZ), shocks play a key role in triggering star formation and driving chemical enrichment. The Sgr B2 complex is a prime template, hosting massive protoclusters (N, M, S) and the northern G+0.693 cloud, which exhibits shock-induced prestellar signatures. We report on G+0.633-0.0604, a newly identified shock-dominated and chemically rich cloud at the southern edge of Sgr B2, where the next star formation episodes are proposed. We characterise its physical properties and the shocks shaping it. We present analyses on CH$3$CCH, CH$_3$CN, HC$_3$N, HNCO and several isotopologues of CO to infer the gas $T{\rm kin}$ and density, using high-sensitivity spectral surveys from the Yebes 40m, IRAM 30m and APEX radio telescopes that covered ~100 GHz across the 31-275 GHz range. We also used 3 mm IRAM 30m mosaics (13'$\times$13') of Sgr B2 in HC$3$N, HNCO and C$_2$H$_5$OH to probe G+0.633 environment. We identify three velocity components: a narrow main one (C1, $v{\rm LSR}$~48.5 km/s; FWHM~10 km/s), and two broader, fainter components at higher velocities, C2 (~61 km/s; ~13 km/s) and C3 (~89 km/s; ~18 km/s), all showing similar properties ($T_{\rm kin}$~55-90 K, $N_{\rm H_2}$~(3-7)$\times$10$^{22}$ cm$^{-2}$, $n_{\rm H_2}$~(0.5-2.5)$\times$10$^{4}$ cm$^{-3}$) and extended distributions. C1 delineates G+0.633 physically and coincides with a peak in HNCO, supporting a shock-driven origin likely rooted in the cloud-cloud collision shaping Sgr B2 and also traced by C2, which extends north to G+0.693. C3 is kinematically unlinked and related to large-scale CMZ dynamics. Of the three, C1 may represent a very early protocluster phase, yet to be confirmed. G+0.633 thus emerges as a new shock-dominated CMZ cloud resembling G+0.693, providing another unique laboratory to investigate how shocks drive molecular complexity and regulate the onset of cluster formation in the CMZ.

• The paper demonstrates that the dominant C1 component in G+0.633 exhibits narrow linewidths and high gas densities, indicating the onset of protocluster collapse.
• It employs multi-frequency spectral surveys and both LTE and non-LTE modeling to derive kinetic temperatures (55–90 K), column densities, and volume densities.
• The analysis highlights shock-driven chemistry as the key mechanism driving enhanced molecular complexity amid suppressed star formation in the CMZ.

Physical Conditions and Astrochemical Context of the G+0.633–0.0604 Molecular Cloud in the Galactic Centre

Introduction and Motivations

The Central Molecular Zone (CMZ) of the Milky Way is an extreme environment characterized by high turbulence, elevated temperatures, and complex kinematics, yet paradoxically exhibits an unusually low star formation rate relative to its molecular gas reservoir. Within the CMZ, Sgr B2 is the preeminent complex in terms of mass and molecular richness, hosting active star-forming protoclusters as well as chemically enigmatic, shock-dominated clouds such as G+0.693–0.027. The discovery and detailed physical characterization of the southern molecular cloud G+0.633–0.0604 (hereafter G+0.633) at the periphery of Sgr B2 provides a vital counterpart to G+0.693 and extends the template for studying shock-driven astrochemical environments within the CMZ.

Figure 1: Overview of the Sgr B2 complex in ALMA 3 mm dust continuum, demonstrating the spatial relation of G+0.633, Sgr B2 protoclusters, and the previously-studied G+0.693.

Observational Campaigns and Methodology

The paper presents a comprehensive suite of broadband spectral line surveys toward G+0.633 using Yebes 40m, IRAM 30m, and APEX (covering 31–275 GHz), augmented by IRAM 30m 3 mm mosaics of Sgr B2. Key molecular probes—CH₃CCH, CH₃CN, HC₃N, HNCO, and CO isotopologues—enable independent diagnostics of kinetic temperature ($T_\mathrm{kin}$), column density ($N_{\mathrm{H}_2}$), and volume density ($n_{\mathrm{H}_2}$) with sensitivity to different gas components. Spectral modeling was performed under LTE using MADCUBA/SLIM, with column densities and excitation temperatures refined with non-LTE radiative transfer (RADEX) for critical density tracers.

Kinematic Decomposition and Physical Characterization

Three discrete velocity components were robustly identified across all key tracers in G+0.633:

• C1 ($v_\mathrm{LSR} \sim 48.5$ km s$^{-1}$, FWHM $\sim 10$ km s$^{-1}$), the dominant, narrowest, and densest feature, physically demarcating G+0.633;
• C2 ($\sim$61 km s$^{-1}$, FWHM $\sim$13 km s$N_{\mathrm{H}_2}$0), broader and fainter, extending northward toward G+0.693;
• C3 ($N_{\mathrm{H}_2}$189 km s$N_{\mathrm{H}_2}$2, FWHM $N_{\mathrm{H}_2}$318 km s$N_{\mathrm{H}_2}$4), the most turbulent and kinematically offset, ascribed to large-scale CMZ flows rather than Sgr B2 proper.

All components show remarkably similar physical properties: $N_{\mathrm{H}_2}$5–90 K, $N_{\mathrm{H}_2}$6–$N_{\mathrm{H}_2}$7 cm$N_{\mathrm{H}_2}$8, and $N_{\mathrm{H}_2}$9–$n_{\mathrm{H}_2}$0 cm$n_{\mathrm{H}_2}$1. The chemical profiles and thermal conditions mirror those seen in the prototypical post-shock G+0.693, but with important distinctions in turbulence and spatial structure.

Figure 2: Transitions of CO isotopologues used to derive the $n_{\mathrm{H}_2}$2 column density, showing the kinematic decomposition into the three main components.

Shock Processes and Astrochemical Diagnostics

The physical segregation of C1 with intense HNCO emission is a critical diagnostic: HNCO has a well-established association with low-velocity ($n_{\mathrm{H}_2}$3–20 km s$n_{\mathrm{H}_2}$4) shocks, and its peak denotes regions of active or recent shock processing. The spatially extended, yet morphologically distinct, emission maps of HC₃N, HNCO, and ethanol further delineate C1 as the locus of shock-driven chemistry in G+0.633. These features are consistent with the theoretical framework of large-scale cloud–cloud collisions that have been proposed to set the evolutionary sequence of Sgr B2 and trigger molecular complexity via grain mantle sputtering and turbulence dissipation.

Comparison to Northern Counterpart G+0.693–0.027

A detailed comparison with G+0.693 is performed. While both regions are shock-enriched and molecularly prolific, G+0.693 exhibits broader dominant velocity components, higher temperatures, and contains well-identified post-shock prestellar condensations and deuterium fractionation. In contrast, G+0.633 displays more uniform, moderate turbulence, and lacks current evidence for advanced prestellar evolution—a potential indicator of an earlier shock-interaction stage. Notably, the C1 component in G+0.633, with its narrower linewidth and higher $n_{\mathrm{H}_2}$5, may signal the onset of collapse toward protocluster formation, although this requires confirmation from high-angular-resolution studies of deuterated and complex-prestellar-tracing molecules.

Broader Implications for Star Formation and Molecular Complexity in the CMZ

The identification of G+0.633 as a southern analog to G+0.693 supports the hypothesis that shock activity at Sgr B2's periphery regulates the spatial and temporal sequence of cluster formation in the CMZ. The suppression of massive star formation at column densities below $n_{\mathrm{H}_2}$6 cm$n_{\mathrm{H}_2}$7, the thermal decoupling between gas ($n_{\mathrm{H}_2}$8 K) and dust ($n_{\mathrm{H}_2}$9 K), and the chemical signatures of efficient mantle desorption all reinforce the notion that environmental shocks are the primary agents driving the molecular inventory and setting the preconditions for cluster assembly under CMZ extremes.

Prospects for Future Astrochemical and Star Formation Studies

The physical characterization in this paper establishes G+0.633 as a benchmark for comparative studies of shock chemistry and pre-stellar evolution in external galactic nuclei. With over 120 molecules identified and an exceptionally rich spectrum, G+0.633 is poised for future investigations of spatially-resolved complex organic molecule distributions, isotopic fractionation, and the time evolution from shock-heated to gravitationally unstable phases. Forthcoming high-resolution, multi-wavelength campaigns will be required to resolve sub-parsec structure, constrain the initial phases of cluster formation, and further probe the interplay between shocks, chemistry, and feedback in the CMZ context.

Conclusion

The authors demonstrate that G+0.633–0.0604 is a shock-dominated, chemically rich molecular cloud that exemplifies the pre-/proto-cluster evolutionary sequence in the Sgr B2 complex. Its gas-phase properties, kinematic structure, and spatial morphology cohere with a scenario of large-scale cloud collision-induced shocks as the primary mechanism for molecular complexity and delayed star formation in the CMZ. G+0.633 now joins G+0.693 as a critical laboratory for the study of astrochemical evolution and the onset of cluster formation under the most extreme conditions of the inner Galaxy.

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References

San Andrés et al., "The Galactic Centre G+0.633–0.0604 molecular cloud: a new astrochemical gold mine. I. Gas physical properties" (2607.01481)