V. Villanueva, M. Blaña, A. D. Bolatto, M. Rubio, E. Tarantino, R. Herrera-Camus, A. Burkert, D. A. D. Vaz, J. I. Read, G. Galaz, C. Muñoz, D. Calderón, M. Behrendt, J. A. Carballo-Bello, E. Gray, M. Fellhauer

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.

Keywords
galaxies: dwarf / galaxies: evolution / galaxies: ISM / Local Group

Astronomy & Astrophysics
Volume 699, Article Number L11, Number of pages 8
2025 July

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