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

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_{\star }$$\sim$10${}^5$ M${}_{\odot }$) known to date. We acquired and present new Atacama Compact Array (ACA) ${}^{12}$CO($J$=1-0) data within our CHIMERA Survey project for the central region of Leo~T, a metal-poor ([M/H]$\sim$-1.7) dwarf in the Milky Way (MW) outskirts. We identified three compact molecular clouds ($<13$ pc) with estimated upper limit virial masses of $M_{\mathrm{mol}}$$\sim$5$\times 10^3$ M${}_{\odot }$ each and a total of 1.4$\pm$0.4$\times$10${}^4$ M${}_{\odot }$, corresponding to $\sim 3\%$ of the total gas mass. We obtained CO-to-H${}_2$ conversion factors (${\alpha }_{\mathrm{CO}}$) as high as $\sim$155 M${}_{\odot }$ $(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$ and mean molecular gas surface densities of ${\Sigma }_{\mathrm{mol}}$$\sim$9 M${}_{\odot }$ pc${}^{-2}$ that are consistent with values found in dwarf galaxies with extremely low metal content. All CO clouds are shifted ($\sim$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 $\Delta v_{\mathrm{los}}\sim +13$ km s${}^{-1}$ being within twice the velocity dispersion ($\Delta v_{\mathrm{los}}/{\sigma }_{\mathrm{HI},\mathrm{los}}\sim 2$) and probably bound. However, the northern cloud is faster ($\Delta v_{\mathrm{los}}\sim +57$ km s${}^{-1}$); our models with low halo masses ($M_{\mathrm{h}}\lesssim 10^9$ M${}_{\odot }$) 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.

References in corpus (23)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The Local Group dwarf Leo T: HI on the brink of star formation
• On the formation of dwarf galaxies and stellar halos
• The structural properties and star formation history of Leo T from deep LBT photometry
• A new all-sky map of Galactic high-velocity clouds from the 21-cm HI4PI survey
• Is Ursa Major II the Progenitor of the Orphan Stream?
• The ALMA Interferometric Pipeline Heuristics
• Observational constraints on stellar feedback in dwarf galaxies
• Carbon monoxide in an extremely metal-poor galaxy
• The Molecular Interstellar Medium of the Local Group Dwarf NGC6822
• Late Gas accretion onto Primordial Minihalos: a Model for Leo T, Dark Galaxies and Extragalactic High-Velocity Clouds
• Solo dwarfs III: Exploring the orbital origins of isolated Local Group galaxies with Gaia Data Release 2
• EDGE: What shapes the relationship between HI and stellar observables in faint dwarf galaxies?
• EDGE: The emergence of dwarf galaxy scaling relations from cosmological radiation-hydrodynamics simulations
• The CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438, and the Phoenix dwarf
• Over-sized gas clumps in an extremely-metal-poor molecular cloud revealed by ALMA's pc-scale maps
• Dwarfs in the Milky Way halo outer rim: first in-fall or backsplash satellites?
• Life and death of a hero - Lessons learned from modeling the dwarf spheroidal Hercules: an incorrect orbit?
• The MUSE-Faint survey. IV. Dissecting Leo T, a gas-rich relic with recent star formation
• The Milky Way satellite galaxy Leo T: A perturbed cored dwarf
• 12CO and 13CO observation of the low-metallicity dwarf galaxy DDO 154