First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H$_2$ ratio

Abstract

Molecular hydrogen (H$_2$) is by far the most abundant molecule in the Universe. However, due to the low emissivity of H$_2$, carbon monoxide (CO) is widely used instead to trace molecular gas in galaxies. The relative abundances of these molecules is expected to depend on both physical (e.g., density) and chemical (e.g., metal enrichment) properties of the gas, making direct measurements in diverse environments crucial. We present a systematic search for CO in absorption toward 34 stars behind H$_2$ gas in the Magellanic Clouds using the Hubble Space Telescope. We report the first two definitive detections of CO absorption in the Large Magellanic Cloud (LMC) and one in the Small Magellanic Cloud (SMC), along with stringent upper limits for the remaining sightlines. Non-detections of CO are consistent with models of low thermal pressures and/or low metallicities while detections at the lower metallicities of the Magellanic Clouds require higher thermal pressures, $P_{\rm th}=10^5-10^6$$\,$K$\,$cm$^{-3}$ than detections the Milky Way at similar $N({\rm H_2})$. Notably, the high density derived from the rotational excitation of CO towards SK$\,$143 in the SMC suggests full molecularization of CO in the absorbing cloud, with CO/H$_2 = 8.3^{+2.0}_{-1.6}\times10^{-5}$ consistent with the standard ratio ($3.2\times10^{-4}$) measured in dense molecular gas in the Milky Way, scaled to the SMC's $0.2\,Z_{\odot}$ metallicity.