Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM
  Understanding stellar birth requires observations of the clouds in which they
form. These clouds are dense and self-gravitating, and in all existing
observations, they are molecular with H_2 the dominant species and CO the best
available tracer. When the abundances of carbon and oxygen are low compared to
hydrogen, and the opacity from dust is also low, as in primeval galaxies and
local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it
is difficult for it to accumulate inside dense clouds. Here we report
interferometric observations of CO clouds in the local group dwarf irregular
galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13% of the
solar value and 50% lower than the previous CO detection threshold. The clouds
are tiny compared to the surrounding atomic and H_2 envelopes, but they have
typical densities and column densities for CO clouds in the Milky Way. The
normal CO density explains why star clusters forming in dwarf irregulars have
similar densities to star clusters in giant spiral galaxies. The low cloud
masses suggest that these clusters will also be low mass, unless some
galaxy-scale compression occurs, such as an impact from a cosmic cloud or other
galaxy. If the massive metal-poor globular clusters in the halo of the Milky
Way formed in dwarf galaxies, as is commonly believed, then they were probably
triggered by such an impact.
