Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM

New and archival observations of the low-metallicity dwarf irregular galaxy WLM show that it contains carbon monoxide, the main tracer for interstellar clouds capable of forming stars, and suggest that in small galaxies both star-forming cores and carbon monoxide become increasingly rare as the metallicity decreases. Star formation in a nearby galaxy Dwarf irregular galaxies such as Wolf–Lundmark–Melotte (WLM) arethe best local analogues for early galaxies available, similar both in size and paucity of elements heavier than helium — referred to as 'metals' by astronomers. Attempts to study these galaxies as models of star formation in the early Universe have been hampered by their apparent lack of carbon monoxide — a key tracer for star-forming regions in galaxies with greater metallicities. This absence of CO has raised doubts over the presumed role of molecules, rather than dust particles, in creating the cold gas necessary for star formation. Now, observations with the APEX telescope have detected CO within two specific star-forming regions in WLM, suggesting that star formation continues in dense molecular gas at lower metallicities than previously observed. The similarity between the metallicities of galaxies such as WLM and those of larger galaxies at high redshift implies that it should be possible to study star formation in young galaxies using the usual techniques. Carbon monoxide (CO) is the primary tracer for interstellar clouds where stars form, but it has never been detected in galaxies in which the oxygen abundance relative to hydrogen is less than 20 per cent of that of the Sun, even though such ‘low-metallicity’ galaxies often form stars. This raises the question of whether stars can form in dense gas without molecules, cooling to the required near-zero temperatures by atomic transitions and dust radiation rather than by molecular line emission 1 ; 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 per cent of the solar value 2 , 3 . We use new submillimetre observations and archival far-infrared observations to estimate the cloud masses, which are both slightly greater than 100,000 solar masses. The clouds have produced stars at a rate per molecule equal to 10 per cent of that in the local Orion nebula cloud. The CO fraction of the molecular gas is also low, a