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, $P_{\mathrm{th}}=10^5-10^6$Kcm${}^{-3}$ than detections the Milky Way at similar $N({\mathrm{H}}_2)$. Notably, the high density derived from the rotational excitation of CO towards SK143 in the SMC suggests full molecularization of CO in the absorbing cloud, with CO/H${}_2=8.3_{-1.6}^{+2.0}\times 10^{-5}$ consistent with the standard ratio ($3.2\times 10^{-4}$) measured in dense molecular gas in the Milky Way, scaled to the SMC's $0.2Z_{\odot }$ metallicity.

10 pages, 8 figures, accepted for publication in A&A Letters

References in corpus (19)

• A distance to the Large Magellanic Cloud that is precise to one per cent
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• A distance determination to the Small Magellanic Cloud with an accuracy of better than 2 percent based on late-type eclipsing binary stars
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• The First Positive Detection of Molecular Gas in a GRB Host Galaxy
• VLT/UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
• Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048
• Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
• Large Metallicity Variations in the Galactic Interstellar Medium
• X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description
• X-shooter observations of strong H${}_2$-bearing DLAs at high redshift
• METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. I. Overview and Initial Results
• HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud
• The Distribution of Metallicities in the Local Galactic Interstellar Medium
• Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ rotational and CI fine-structure levels
• Discovery of CO absorption at z=0.05 in G0248+430
• The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus
• Warm Molecular Gas in the Central Parsecs of the Buried Nucleus of NGC 4418 Traced with the Fundamental CO Ro-vibrational Absorptions