Carbon Monoxide Clouds at Low-Metallicity in the WLM Galaxy
  Carbon monoxide (CO) is the primary tracer for interstellar clouds where
stars form, yet CO has never been detected in galaxies with an Oxygen abundance
relative to Hydrogen less than 20% of solar, even though such low metallicity
galaxies often form stars. This raises the question of whether stars can form
in dense gas without the usual molecules, cooling to the required near-zero
temperatures by atomic transitions and dust radiation rather than molecular
line emission (Krumholz 2012), and it highlights uncertainties about star
formation in the early universe, when the metallicity was generally low. Here
we report the detection of CO in two regions of a local dwarf irregular galaxy,
WLM, where the metallicity is 13% of the solar value (Lee et al. 2005, Asplund
et al. 2009). New sub-millimeter observations and archival far-infrared
observations are used to estimate the cloud masses, which are both slightly
larger than 10^5 Msun. The clouds have produced stars at a rate per molecule
equal to 10% of that in the local Orion Nebula cloud. The CO fraction of the
molecular gas is also low, about 3% of the Milky Way value. These results
suggest that both star-forming cores and CO molecules become increasingly rare
in H_2 clouds as the metallicity decreases in small galaxies.
