When the universe became dusty

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Science  24 Jun 2016:
Vol. 352, Issue 6293, pp. 1520
DOI: 10.1126/science.aaf9761

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Observations of the most distant galaxies known are now reaching into the epoch when the first generations of stars were being formed. As stars are the main factories of elements heavier than hydrogen and helium, one also expects to see a reduced abundance of these heavy elements and of the dust that condenses out of them. Recent observations of galaxies within 1 billion years of the Big Bang have shown that the far-infrared (far-IR) emission from dust in these galaxies indeed becomes fainter. Also, the usually strong far-IR emission line from ionized carbon remains undetected in an increasing number of galaxies of redshift z > 7 (13). Hence, it has been assumed that detailed studies of the interstellar medium (ISM) in these galaxies will be very challenging, even with the powerful Atacama Large Millimeter/submillimeter Array (ALMA). On page 1559 of this issue, Inoue et al. (4) detect doubly ionized oxygen at a rest wavelength of 88 µm from a galaxy at z = 7.2, where neither dust nor ionized carbon was detected. The oxygen to far-ultraviolet luminosity ratio in this galaxy is similar to nearby dwarf galaxies with an oxygen abundance of 10 to 60% that of the Sun (5), which suggests that some substantial chemical enrichment has already occurred. However, the similarities stop there; in dwarf galaxies, the dust and ionized carbon lines are not as faint. It appears that the dust in this young galaxy may not have formed yet, or that it was destroyed, for example, by supernova shock waves.