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

Understand

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.