Cospatial 21 cm and metal-line absorbers in the epoch of reionization -- I : Incidence and observability

TL;DR

This study uses high-resolution simulations to explore the occurrence and observability of metal and 21 cm absorption lines during reionization, identifying conditions for their detection and implications for understanding early universe gas properties.

Contribution

It introduces a detailed analysis of aligned metal and 21 cm absorbers, classifies their types, and predicts their observability with future telescopes during reionization.

Findings

Aligned and cospatial absorbers originate from overdense structures.

Incidence peaks at redshift z ≈ 8 for aligned and cospatial absorbers.

Future telescopes like ELT and SKA1-LOW can detect these absorbers during reionization.

Abstract

Overdense, metal-rich regions, shielded from stellar radiation might remain neutral throughout reionization and produce metal as well as 21 cm absorption lines. Simultaneous absorption from metals and 21 cm can complement each other as probes of underlying gas properties. We use Aurora, a suite of high resolution radiation-hydrodynamical simulations of galaxy formation, to investigate the occurrence of such "aligned" absorbers. We calculate absorption spectra for 21 cm, OI, CII, SiII and FeII. We find velocity windows with absorption from at least one metal and 21 cm, and classify the aligned absorbers into two categories: 'aligned and cospatial absorbers' and 'aligned but not cospatial absorbers'. While 'aligned and cospatial absorbers' originate from overdense structures and can be used to trace gas properties, 'aligned but not cospatial absorbers' are due to peculiar velocity effects. The incidence of absorbers is redshift dependent, as it is dictated by the interplay between reionization and metal enrichment, and shows a peak at $z \approx 8$ for the aligned and cospatial absorbers. While aligned but not cospatial absorbers disappear towards the end of reionization because of the lack of an ambient 21 cm forest, aligned and cospatial absorbers are associated with overdense pockets of neutral gas which can be found at lower redshift. We produce mock observations for a set of sightlines for the next generation of telescopes like the ELT and SKA1-LOW, finding that given a sufficiently bright background quasar, these telescopes will be able to detect both types of absorbers throughout reionization.