Intense star formation within resolved compact regions in a galaxy at z = 2.3

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Abstract
The individual star-forming regions of the massive galaxies in the early Universe, at redshifts of around z = 2, are beyond the reach of even the largest of today's telescopes. But with help from gravitational lensing by a massive intervening galaxy cluster, brightening the image 32-fold and making it look larger, intense star formation has been observed in the submillimetre galaxy SMMJ2135-0102 at redshift z = 2.3259. The results reveal luminosity densities comparable to those of the dense cores of giant molecular clouds in the local Universe, suggesting that the underlying physics of star formation is similar to that of nearby galaxies, though the regions in the distant galaxies are about 100 times larger and 107 times more luminous overall. Massive galaxies in the early Universe have been shown to be forming stars at high rates. Probing the properties of individual star-forming regions is beyond the resolution and sensitivity of existing telescopes. Here, however, observations are reported of the galaxy SMMJ2135–0102 at redshift z=2.3259, which has been gravitationally magnified by a factor of 32 by a galaxy cluster lens in the foreground. The physics underlying star formation here is similar to that in local galaxies, but the energetics are very different. Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates1,2,3. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre wavelengths and which may be forming stars at a rate of 1,000 solar masses (M⊙) per year4,5,6,7. These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies8,9. Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 107 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z ≈ 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.
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