Inefficient star formation in extremely metal poor galaxies

Spatially resolved infrared observations of two galaxies with oxygen abundances below 10 per cent of the solar value show that stars formed very inefficiently in seven star-forming clumps, suggesting that star formation may have been very inefficient in the early Universe. A model for early star formation The first generations of stars formed from gas containing few or no elements heavier than helium (called 'metals' by astronomers). Star formation processes under these conditions would probably have been very different from those seen in galaxies today. Now, using the far-infrared capabilities of the Herschel Space Observatory, Yong Shi et al . have mapped the dusty regions of two nearby young galaxies where the extreme conditions resemble those prevailing in the primordial galaxies in the early Universe. They estimate the dust/gas ratio in regions dominated by atomic gas and use that to estimate the amount of gas in the star-forming regions. They find star formation that is only a tenth as efficient as that seen in normal, metal-rich galaxies today. This work provides a glimpse of how stars may have formed in the early Universe and a convenient model in which theoretical models for early galaxy evolution can be put to the test. The first galaxies contain stars born out of gas with few or no ‘metals’ (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation 1 , 2 , but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun 2 , 3 , 4 . Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities 5 , 6 , 7 , and while dust has been used to trace gas in low-metallicity galaxies 5 , 8 , 9 , 10 , low spatial resolution in the far-infrared has typically led to large uncertainties 9 , 10 . Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.