Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR

TL;DR

This paper evaluates LOFAR's potential to detect the 21cm forest from the diffuse intergalactic medium, highlighting challenges in distinguishing reionization models and using absorption features to probe the universe's thermal history.

Contribution

It provides simulation-based analysis of 21cm forest detectability with LOFAR, emphasizing the impact of X-ray backgrounds and reionization history on observational signatures.

Findings

Similar reionization models produce nearly indistinguishable spectra.

Absence of 21cm absorption could indicate a strong X-ray background.

Detectable absorption features at high redshift and along specific lines of sight.

Abstract

We discuss the feasibility of the detection of the 21cm forest in the diffuse IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been derived using simulations of reionization which include detailed radiative transfer of ionizing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV vs. x-ray heating) we do not expect to distinguish them by means of observations of the 21cm forest. Because the presence of a strong x-ray background would make the detection of 21cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the x-ray background and the thermal history of the universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z>12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high density pockets of gas could be detected also at z~7 along preferential lines of sight.