Abstract
Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies such as our Milky Way) remains unclear. Here we report the detection of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with 7% solar metallicity, with additional detections in two galaxies at higher metallicities. Such detections offer direct evidence for the existence of molecular gas in these galaxies that contain few metals. Using archived infrared data, it is shown that the molecular gas mass per CO luminosity at extremely low metallicity is approximately one-thousand times the Milky Way value.
Highlights
Metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe
There is an indirect evidence for the presence of molecular gas in these galaxies[5,6,7], the emission from the molecule carbon monoxide (CO), which is the primary tracer of molecular gas, has never been detected in them[8,9,10,11,12,13]
We report the detection of CO in a galaxy at 7% of solar metallicity, along with additional detections in galaxies at 13% and 18% solar metallicity; these data offer direct evidence for the existence of molecular gas in these metal-poor galaxies
Summary
Metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. The CO luminosity is related to both far-infrared luminosities and SFRs among massive star-forming galaxies, indicating that the molecular gas mass as traced by CO is related to star-formation activities.
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