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

TL;DR

This study reports the first detection of CO molecules in Leo T, the lowest mass galaxy with cold molecular gas, revealing insights into molecular gas presence, dynamics, and star formation in extremely low-metallicity dwarf galaxies.

Contribution

It provides the first CO detection in Leo T, demonstrating the presence and properties of molecular gas in the lowest mass gas-rich dwarf galaxy.

Findings

Detected three compact molecular clouds in Leo T.

High CO-to-H2 conversion factors indicating low metallicity conditions.

Evidence of molecular gas being expelled, influencing star formation and galaxy evolution.

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}$$\sim$10$^5$ M$_{\odot}$) known to date. We acquired and present new Atacama Compact Array (ACA) $^{12}$CO($J$=1-0) data within our CHIMERA Survey project for the central region of Leo~T, a metal-poor ([M/H]$\sim$-1.7) dwarf in the Milky Way (MW) outskirts. We identified three compact molecular clouds ($<13$ pc) with estimated upper limit virial masses of $M_{\rm mol}$$\sim$5$\times10^{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 ($\alpha_{\rm CO}$) as high as $\sim$155 M$_{\odot}$ $({\rm K\, km\, s^{-1}\, pc^2})^{-1}$ and mean molecular gas surface densities of $\Sigma_{\rm 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 $\Delta v_{\rm los}\sim\!+13$ km s$^{-1}$ being within twice the velocity dispersion ($\Delta v_{\rm los}/\sigma_{\rm HI,los}\sim2$) and probably bound. However, the northern cloud is faster ($\Delta v_{\rm los}\sim\!+57$ km s$^{-1}$); our models with low halo masses ($M_{\rm h}\! \lesssim \!10^9$ 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.