A glimpse of the end of the dark ages: the gamma-ray burst of 23 April
  2009 at redshift 8.3
  It is thought that the first generations of massive stars in the Universe
were an important, and quite possibly dominant, source of the ultra-violet
radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a
state in which it has remained to the present day. Measurements of cosmic
microwave background anisotropies suggest that this phase-change largely took
place in the redshift range z=10.8 +/- 1.4, while observations of quasars and
Lyman-alpha galaxies have shown that the process was essentially completed by
z=6. However, the detailed history of reionization, and characteristics of the
stars and proto-galaxies that drove it, remain unknown. Further progress in
understanding requires direct observations of the sources of ultra-violet
radiation in the era of reionization, and mapping the evolution of the neutral
hydrogen fraction through time. The detection of galaxies at such redshifts is
highly challenging, due to their intrinsic faintness and high luminosity
distance, whilst bright quasars appear to be rare beyond z~7. Here we report
the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07.
This is well beyond the redshift of the most distant spectroscopically
confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive
stars were being produced, and dying as GRBs, ~625 million years after the Big
Bang. In addition, the accurate position of the burst pinpoints the location of
the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will
constrain the evolving rate of star formation in the early universe, while
rapid spectroscopy of their afterglows will allow direct exploration of the
progress of reionization with cosmic time.
