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Galaxies

A long-distance record breaker

Nature volume 398, pages 558–559 (1999)Cite this article

Direct observations of the earliest luminous objects are only now beginning to illuminate the study of the origin and formation of galaxies. Each step beyond the previous limit of discovery holds the promise that new techniques will reveal the precursors of the galaxies we see today. On page 586 of this issue, Chen, Lanzetta and Pascarelle1 report the discovery of an object that probably has the largest redshift ever measured, z = 6.68. In the model of the expanding Universe, high redshifts — detected as shifts in spectral lines to longer wavelengths — of galaxies and quasars directly indicate their great distances from Earth. A redshift of 6.68 corresponds to a time when the Universe was only 5% of its present age. The previous record holder, reported last year2, was found at a distance or ‘look-back’ time corresponding to when the Universe was 1% older.

Three factors have made the discovery of such high-redshift galaxies particularly difficult. First, such objects are not merely dimmed with distance by the cosmological equivalent to the inverse square law; galaxies are also extended, and surface brightness is dimmed by (1 + z)4, so, only the brightest clumps in a system will be detectable above the sky and detector background. Second, there is no redshifted signal from the ultraviolet to the red part of the spectrum because the flux is completely absorbed by hydrogen in intergalactic clouds. Finally, the light produced and emitted in spectral lines by hydroxyl ions in the Earth's own atmosphere can swamp faint signals emitted in the far red by the most distant objects.

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Figure 1: Expected spectral signature from a galaxy with a redshift of 6.

KITT PEAK NATIONAL OBSERVATORY

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  1. Kitt Peak National Observatory , Tucson, 85726-6732, Arizona, USA

    Richard Green

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  1. Richard Green
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Green, R. A long-distance record breaker. Nature 398, 558–559 (1999). https://doi.org/10.1038/19190

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