[Paper Review] The 21 cm Forest: Radio Absorption Spectra as a Probe of the Intergalactic Medium Before Reionization

Steven R. Furlanetto, Abraham Loeb|arXiv (Cornell University)|Jun 18, 2002

Radio Astronomy Observations and Technology4 citations

TL;DR

This paper proposes using 21 cm radio absorption spectra toward high-redshift radio sources to probe the intergalactic medium (IGM) before reionization. By modeling absorption from minihalos and protogalactic disks, it demonstrates that next-generation telescopes like the SKA and LOFAR can detect these features, enabling measurement of the IGM temperature and radiation background evolution, with GRB host galaxy absorption offering a spectroscopic redshift probe.

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 semi-analytic 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 also cause much 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 HI in the host galaxy of a GRB should be observable, offering a route to spectroscopically determine the burst redshift.

Motivation & Objective

• To investigate the detectability of 21 cm absorption features from neutral hydrogen in the intergalactic medium before cosmic reionization.
• To assess the observability of absorption features from minihalos and protogalactic disks using next-generation low-frequency radio telescopes.
• To explore how absorption statistics along lines of sight to radio-loud quasars can constrain the evolution of the IGM temperature and radiation background.
• To evaluate the potential for detecting HI absorption in the host galaxies of gamma-ray bursts (GRBs) as a spectroscopic redshift diagnostic.

Proposed method

• Using semi-analytic methods to model the distribution and properties of minihalos and protogalactic disks in the pre-reionization IGM.
• Computing the number density of observable 21 cm absorption features along lines of sight toward high-redshift radio sources.
• Assessing the detectability of these features under realistic conditions, including source brightness and telescope sensitivity.
• Evaluating the depth and detectability of absorption features from both minihalos and protogalactic disks in radio and gamma-ray burst spectra.
• Modeling the expected signal-to-noise ratio for absorption features in the context of upcoming low-frequency arrays like LOFAR and SKA.

Experimental results

Research questions

• RQ1 Can 21 cm absorption features from minihalos and protogalactic disks be detected by next-generation low-frequency radio telescopes?
• RQ2 How do the statistics of minihalo absorption features along quasar lines of sight constrain the evolution of the IGM temperature and radiation background?
• RQ3 What is the expected depth and detectability of absorption features from protogalactic disks in radio-loud quasars and gamma-ray burst afterglows?
• RQ4 Can HI absorption in the host galaxy of a GRB be observed to provide a spectroscopic redshift measurement?

Key findings

• Minihalo absorption features are statistically abundant and should be detectable by the Low Frequency Array and the Square Kilometer Array, provided sufficiently bright background radio sources exist.
• Protogalactic disk absorption features are rarer but produce much deeper absorption, making them potentially visible in the spectra of both radio-loud quasars and gamma-ray burst afterglows.
• The statistics of minihalo absorption along quasar lines of sight offer a viable method to measure the evolution of the IGM temperature and the cosmic radiation background over time.
• The 21 cm absorption feature from HI in the host galaxy of a gamma-ray burst is expected to be observable, providing a direct spectroscopic route to determine the burst's redshift.