A glimpse of the end of the dark ages: the gamma-ray burst of 23 April 2009 at redshift 8.3

TL;DR

The discovery of a gamma-ray burst at redshift 8.3 provides direct evidence of massive star formation and offers a new way to study the universe's reionization era, which occurred roughly 625 million years after the Big Bang.

Contribution

This paper reports the first detection of a gamma-ray burst at such a high redshift, extending the observational window into the early universe and star formation history.

Findings

Detected GRB 090423 at z=8.26, the most distant known object of its kind.

Confirmed massive star formation occurred within 625 million years after the Big Bang.

Pinpointed the location of the most distant galaxy known to date.

Abstract

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.