Prospects of a statistical detection of the 21-cm forest and its potential to constrain the thermal state of the neutral IGM during reionization

TL;DR

This paper models the 21-cm forest signal during reionization to evaluate its detectability with current and future radio telescopes, showing potential to constrain the IGM's thermal and ionization states at redshift 6.

Contribution

It presents semi-numerical simulations of the 21-cm forest signal incorporating instrumental effects, assessing detection prospects and constraints on the IGM at z=6.

Findings

Detection of the 1D power spectrum is feasible at large scales with uGMRT and SKA1-low.

Null-detections can place tighter constraints on IGM thermal and ionization states.

A null-detection with 50 hr uGMRT observations can challenge models of cold, neutral IGM at z=6.

Abstract

The 21-cm forest signal is a promising probe of the Epoch of Reionization complementary to other 21-cm line observables and Ly$\alpha$ forest signal. Prospects of detecting it have significantly improved in the last decade thanks to the discovery of more than 30 radio-loud quasars at these redshifts, upgrades to telescope facilities, and the notion that neutral hydrogen islands persist down to $z\lesssim 5.5$. We forward-model the 21-cm forest signal using semi-numerical simulations and incorporate various instrumental features to explore the potential of detecting the 21-cm forest at $z=6$, both directly and statistically, with the currently available (uGMRT) and forthcoming (SKA1-low) observatories. We show that it is possible to detect the 1D power spectrum of the 21-cm forest spectrum, especially at large scales of $k\lesssim8.5\,\rm MHz^{-1}$ with the $500\,\rm hr$ of the uGMRT time and $k\lesssim32.4\,\rm MHz^{-1}$ with the SKA1-low over $50\,\rm hr$ if the intergalactic medium (IGM) is $25\%$ neutral and these neutral hydrogen regions have a spin temperature of $\lesssim30\,\rm K$. On the other hand, we infer that a null-detection of the signal with such observations of 10 radio-loud sources at $z\approx6$ can be translated into constraints on the thermal and ionization state of the IGM which are tighter than the currently available measurements. Moreover, a null-detection of the 1D 21-cm forest power spectrum with only $50\,\rm hr$ of the uGMRT observations of 10 radio-loud sources can already be competitive with the Ly$\alpha$ forest and 21-cm tomographic observations in disfavouring models of significantly neutral and cold IGM at $z=6$.