• DOI:10.1038/443151a
• Corpus ID: 28977650

Astronomy: Dawn after the dark age

@article{McMahon2006AstronomyDA,
  title={Astronomy: Dawn after the dark age},
  author={Richard A. McMahon},
  journal={Nature},
  year={2006},
  volume={443},
  pages={151-152}
}

• Richard A. McMahon
• Published 14 September 2006
• Physics
• Nature

The latest surveys provide evidence for one, maybe two, galaxies farther back in cosmic time than ever detected before. But does the fact that we don't see more mean these are the very first galaxies to be formed?The oldest known galaxyThe galaxy described on page 186 may be, for the moment, the most distant and hence oldest galaxy known. Large samples of galaxies have been found at redshifts of z∼6, but detections at earlier times tend to be uncertain and unreliable. But this 'new' old galaxy has a spectroscopic redshift of z=6.96, corresponding to just 750 million years after the Big Bang; and a Lyman-α emission line in its spectrum suggests that active star formation was under way when the Universe was only about 6% of its present age. This galaxy was detected during a survey using the Subaru Suprime-Cam on the summit of Mauna Kea. Looking at the galaxy population as a whole, the same survey produced a number density of galaxies at z≈7 that is only 18-36% that at z=6.6. A separate search for galaxies at at z∼7–8 using data from the Hubble Space Telescope yielded (conservatively) only one candidate galaxy, where 10 would be expected if there were no evolution in the galaxy population between z∼7 and z∼6. The simplest explanation for this is that the Universe is just too young to have built up many luminous galaxies at z∼7–8 by hierarchical merging of small galaxies.