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 formation1,2, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun2,3,4. 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 metallicities5,6,7, and while dust has been used to trace gas in low-metallicity galaxies5,8,9,10, low spatial resolution in the far-infrared has typically led to large uncertainties9,10. 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.
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Y.S. acknowledges support for this work from the Natural Science Foundation of China (NSFC), grant 11373021, the Strategic Priority Research Program ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences (CAS), grant XDB09000000, and Nanjing University grant 985. Y.G. acknowledges support from the NSFC (grants 11173059 and 11390373) and from the CAS Program (grant XDB09000000). J.W. was supported by the National 973 programme (grant 2012CB821805) and by the NSFC (grant 11173013). Z.-Y.Z. acknowledges support from the European Research Council (ERC) in the form of advanced grant COSMICISM. Q.G. was supported by the NSFC (11273015 and 11133001) and by the National 973 programme (grant 2013CB834905). We thank F. Bigiel for making his data points available to plot contours in Fig. 3, S. P. C. Peters for making available his H i gas map of ESO 146-G14 to us, and L. Piazzo for help in Herschel data reduction. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. This work was supported in part by the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. It was also supported in part by a NASA Herschel grant (OT2_yshi_3) issued by JPL/Caltech.
The authors declare no competing financial interests.
Extended data figures and tables
a, Images of Sextans A in (left to right) the far-ultraviolet, H i gas, 70 µm, 160 µm and 250 µm dust emission. The large circle is the star-forming disk, small circles are star-forming clumps, and ellipses are diffuse regions. b, Images of ESO 146-G14: wavebands and disks/ellipses as in a.
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Shi, Y., Armus, L., Helou, G. et al. Inefficient star formation in extremely metal poor galaxies. Nature 514, 335–338 (2014). https://doi.org/10.1038/nature13820
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