Unveiling the 3D structure of the central molecular zone from stellar kinematics and photometry: The 50 and 20 km/s clouds

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Como Lake Centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁶ Institute of Space Sciences & Astronomy, University of Malta, Msida MSD 2080, Malta
⁷ Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 181-0015, Japan
⁸ Observatorio Astronómico de Quito, Observatorio Astronómico Nacional, Escuela Politécnica Nacional, 170403 Quito, Ecuador
⁹ Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
¹¹ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹² Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA
¹³ Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA
¹⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹⁵ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
¹⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁷ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Study at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹⁸ Cosmic Origins Of Life (COOL) Research DAO, https://coolresearch.io
¹⁹ Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
²⁰ University of Connecticut, Department of Physics, 196A Hillside Road, Unit 3046, Storrs, CT 06269-3046, USA
²¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
²² State Key Laboratory of Radio Astronomy and Technology, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
²³ 23European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile
²⁴ 24Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile
²⁵ Departamento de Astronomía, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
²⁶ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
²⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
²⁸ Institute of Space Studies of Catalonia (IEEC), 08860 Barcelona, Spain
²⁹ Department of Physics and Astronomy, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA

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Received: 20 June 2025
Accepted: 10 December 2025

Abstract

Context. The central molecular zone (CMZ), surrounding the Galactic centre, is the largest reservoir of dense molecular gas in the Galaxy. Despite its relative proximity, the 3D structure of the CMZ remains poorly constrained, primarily due to projection effects.

Aims. We aim to constrain the line-of-sight location of two molecular clouds in the CMZ - the 50 and 20 km/s clouds - and to investigate their possible physical connection using stellar kinematics and photometry. This study serves as a pilot for future applications across the full CMZ.

Methods. We estimated the line-of-sight position of the clouds by analysing stellar kinematics, stellar densities, and stellar populations towards the cloud regions and a control field.

Results. We find an absence of westward moving stars in the cloud regions, which indicates that they lie on the near side of the CMZ. This interpretation is supported by the stellar density distributions. The similar behaviour observed in the two clouds, as well as in the region between them (the ridge), suggests that they are located at comparable distances and are physically linked. We also identified an intermediate-age stellar population (2-7 Gyr) in both regions, consistent with that observed on the near side of the CMZ. We estimated the line-of-sight distances at which the clouds and the ridge become kinematically detectable (i.e. where the proper motion component parallel to the Galactic plane differs from that of the control field at the 3σ level) by converting their measured proper motions parallel to the Galactic plane using a theoretical model of the stellar distribution. We find that the 50 and 20 km/s clouds are located at 43 ± 8 pc and 56 ± 11 pc from Sgr A^*, respectively, and that the ridge lies at 56 ± 11 pc; this supports the idea that the clouds are physically connected through the ridge.