Magnification of light from many distant quasars by gravitational lenses

Exceptionally bright quasars with redshifts up to z = 6.28 have recently been discovered. Quasars are thought to be powered by the accretion of gas onto supermassive black holes at the centres of galaxies. Their maximum (Eddington) luminosity depends on the mass of the black hole, and the brighter quasars are inferred to have black holes with masses of more than a few billion solar masses. The existence of such massive black holes poses a challenge to models for the formation of structures in the early Universe, as it requires their formation within one billion years of the Big Bang. Here we show that up to one-third of known quasars with z approximately equal to 6 will have had their observed flux magnified by a factor of ten or more, as a consequence of gravitational lensing by galaxies along the line of sight. The inferred abundance of quasar host galaxies, as well as the luminosity density provided by the quasars, has therefore been substantially overestimated.     

Suppression of dwarf galaxy formation by cosmic reionization

A large number of faint galaxies, born less than a billion years after the Big Bang, have recently been discovered. Fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of megaparsecs, as observed along the lines of sight to the earliest known quasars. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized, leading to a drop in the cosmic star-formation rate. Here we report evidence for this suppression. We show that the post-reionization galaxies that produced most of the ionizing radiation at a redshift z approximately 5.5 must have had a mass in excess of approximately 10(10.9 +/- 0.5) solar masses (M(o)) or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (approximately 10(8) M(o)). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, before the epoch of dwarf galaxy suppression.     

A large neutral fraction of cosmic hydrogen a billion years after the Big Bang

The fraction of ionized hydrogen left over from the Big Bang provides evidence for the time of formation of the first stars and quasar black holes in the early Universe; such objects provide the high-energy photons necessary to ionize hydrogen. Spectra of the two most distant known quasars show nearly complete absorption of photons with wavelengths shorter than the Lyman alpha transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift of z approximately 6.3, about one billion years after the Big Bang. Here we show that the IGM surrounding these quasars had a neutral hydrogen fraction of tens of per cent before the quasar activity started, much higher than the previous lower limits of approximately 0.1 per cent. Our results, when combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination therefore suggest the presence of a second peak in the mean ionization history of the Universe.     

A characteristic size of approximately 10 Mpc for the ionized bubbles at the end of cosmic reionization

The first galaxies to appear in the Universe at redshifts z > 20 created ionized bubbles in the intergalactic medium of neutral hydrogen left over from the Big Bang. The ionized bubbles grew with time, surrounding clusters of dwarf galaxies and eventually overlapped quickly throughout the Universe over a narrow redshift interval near z approximately 6. This event signalled the end of the reionization epoch when the Universe was a billion years old. Measuring the size distribution of the bubbles at their final overlap phase is a focus of forthcoming programmes to observe highly redshifted radio emission from atomic hydrogen. Here we show that the combined constraints of cosmic variance and light travel time imply an observed bubble size at the end of the overlap epoch of approximately 10 physical Mpc, and a scatter in the observed redshift of overlap along different lines-of-sight of approximately 0.15. This scatter is consistent with observational constraints from recent spectroscopic data on the farthest known quasars. This implies that future radio experiments should be tuned to a characteristic angular scale of 0.5 degrees and have a minimum frequency bandwidth of approximately 8 MHz for an optimal detection of 21-cm flux fluctuations near the end of reionization.