Redshifted 21 Centimeter Signatures around the Highest Redshift Quasars

The Ly-alpha absorption spectrum of the highest redshift quasars indicatesthat they are surrounded by giant HII regions, a few Mpc in size. The neutralgas around these HII regions should emit 21cm radiation in excess of the CosmicMicrowave Background, and enable future radio telescopes to measure thetransverse extent of these HII regions. At early times, the HII regions expandwith a relativistic speed. Consequently, their measured sizes along theline-of-sight (via Ly-alpha absorption) and transverse to it (via 21 cmemission) should have different observed values due to relativistic time-delay.We show that the combined measurement of these sizes would directly constrainthe neutral fraction of the surrounding intergalactic medium (IGM) as well asthe quasar lifetime. Based on current number counts of luminous quasars at z>6,an instrument like LOFAR should detect >2 redshifted 21cm shells per field(with a radius of 11 degrees) around active quasars as bright as those alreadydiscovered by SDSS, and >200 relic shells of inactive quasars per field. Weshow that Ly-alpha photons from the quasar are unable to heat the IGM or tocouple the spin and kinetic temperatures of atomic hydrogen beyond the edge ofthe HII region. The detection of the IGM in 21cm emission around high redshiftquasars would therefore gauge the presence of a cosmic Ly-alpha backgroundduring the reionization epoch.Comment: 11 pages, 6 figures. Submitted to Ap