Abstract
We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semianalytic methods, we compute the number density of observable features caused by both minihalos (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common but cause significantly deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by H I in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift.
Highlights
Despite its apparent simplicity, effective observational probes of the intergalactic medium (IGM) at high redshifts are difficult to find
As we demonstrate 21 cm absorption spectra probe the radiation background and thermal state of the IGM at high redshifts and could be instrumental in testing models of structure formation and reionization
We have shown that absorption at the 21 cm transition of neutral hydrogen in both minihalos and protogalactic disks at high-redshifts can cause non-negligible absorption in the low-frequency spectra of high-redshift radio sources
Summary
Effective observational probes of the intergalactic medium (IGM) at high redshifts are difficult to find. Carilli et al (2002) have pointed out that luminous high-redshift radioloud quasars serve as ideal background sources against which absorption by intervening gas can be seen: with a bright background source it becomes much easier to identify the weak absorption features expected to be produced by the IGM Along such lines of sight one can map the “21 cm forest” of redshifted hyperfine absorption lines and study the neutral IGM over the redshift range 6 z 10. Their properties are sensitive to the uncertain presence of molecular hydrogen; if H2 forms in sufficient quantities it can act as a cooling channel for these minihalos (Barkana & Loeb 2001, and references therein), strongly suppressing their number density (and increasing the global star formation rate) Detection of these halos would provide important insights into the physics of hierarchical structure formation prior to reionization.
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