Probing Reionization with Intensity Mapping of Molecular and Fine Structure Lines

TL;DR

This paper explores the potential of intensity mapping of molecular and fine structure lines, particularly CO and CII, to study the distribution of gas during reionization at redshifts 6 to 8, using interferometric observations to detect fluctuations and cross-correlations.

Contribution

It proposes a novel observational approach combining 21-cm and molecular line intensity mapping to probe the reionization era and the distribution of intergalactic and molecular gas.

Findings

Mean brightness temperature of 0.5 μK at z~6-8

Detectable intensity fluctuations at 30 arcminute scales

Feasibility of cross-correlation to mitigate systematics

Abstract

We propose observations of the molecular gas distribution during the era of reionization. At $z \sim $ 6 to 8, $^{12}$CO$(J=1-0)$ line intensity results in a mean brightness temperature of about 0.5 $\mu$K with rms fluctuations of 0.1 $\mu$K at 1 to 10 Mpc spatial scales, corresponding to 30 arcminute angular scales. This intensity fluctuations can be mapped with an interferometer, similar to existing and planned 21-cm background experiments, but operating at $\sim$ 12 to 17 GHz. We discuss the feasibility of detecting the cross-correlation between HI and CO molecular gas since such a cross-correlation has the advantage that it will be independent of systematics and most foregrounds in each of the 21-cm and CO$(1-0)$ line experiments. Additional instruments tuned to higher-order transitions of the CO molecule or an instrument operating with high spectral resolution at mm-wavelengths targeting 158 $\mu$m CII could further improve the reionization studies with molecular gas. The combined 21-cm and CO line observations has the potential to establish the relative distribution of gas in the inter-galactic medium and molecular gas that are clumped in individual first-light galaxies that are closely connected to the formation of massive stars in these galaxies.