Letter to the Editor

The CHIMERA Survey: The first CO detection in Leo T, the lowest mass known galaxy still hosting cold molecular gas

¹ Departamento de Astronomía, Universidad de Concepción, Avenida Esteban Iturra s/n, Casilla 160-C, Concepción, Chile
² Vicerrectoría de Investigación y Postgrado, Universidad de La Serena, La Serena 1700000, Chile
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, D-85748 Garching bei München, Germany
⁷ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstraße 1, D-81679 München, Germany
⁸ Excellence Cluster ORIGINS, Boltzmannstr 2, D-85748 Garching bei München, Germany
⁹ Instituto de Astrofísica e Ciências do Españo, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁰ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
¹¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹² Departamento de Astronomía, Facultad de Ciencias, Universidad de La Serena. Av. Raul Bitran 1305, La Serena, Chile
¹³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹⁴ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
¹⁵ Millennium Nucleus for Galaxies (MINGAL), Chile
¹⁶ Departamento de Física e Astronomía, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, PT4169-007 Porto, Portugal

^⋆⋆ Corresponding authors: vvillanueva@astro-udec.cl; matias.blana.astronomy@gmail.com

Received: 22 May 2025
Accepted: 1 July 2025

Abstract

We report the first CO detection in Leo T, representing the most extreme observation of carbon monoxide molecules in the lowest stellar mass gas-rich dwarf galaxy (M_⋆ ∼ 10⁵ M_⊙) known to date. We acquired and present new Atacama Compact Array (ACA) ¹²CO(J = 1–0) data within our CHIMERA Survey project for the central region of Leo T, a metal-poor ([M/H] ∼ −1.7) dwarf in the Milky Way (MW) outskirts. We identified three compact molecular clouds (< 13 pc) with estimated upper limit virial masses of M_mol ∼ 5 × 10³ M_⊙ each and a total of 1.4 ± 0.4 × 10⁴ M_⊙, corresponding to ∼3% of the total gas mass. We obtained CO-to-H₂ conversion factors (α_CO) as high as ∼ 155 M_⊙(K km s⁻¹ pc²)⁻¹ and mean molecular gas surface densities of Σ_mol ∼ 9 M_⊙ pc⁻² that are consistent with values found in dwarf galaxies with extremely low metal content. All CO clouds are shifted (∼60 pc) from the stellar population centers, and only one cloud appears within the densest HI region. Two clouds have velocity offsets with the HI of Δv_los ∼ + 13 km s⁻¹ being within twice the velocity dispersion (Δv_los/σ_HI, los ∼ 2) and probably bound. However, the northern cloud is faster (Δv_los ∼ + 57 km s⁻¹); our models with low halo masses (M_h ≲ 10⁹ M_⊙) result in unbound orbits, suggesting that this material is likely being expelled from the dwarf, providing evidence for molecular gas depletion. These properties reveal a perturbed dynamics intertwined with star formation processes in low-mass dwarf galaxies, supporting a scenario of episodic bursts until they are fully quenched by the MW environment.