Mapping the Universe in HD

TL;DR

This paper assesses the potential of line intensity mapping of hydrogen deuteride (HD) to probe the early universe, especially during reionization and cosmic dawn, highlighting future prospects and current limitations.

Contribution

It explores the feasibility of using HD line intensity mapping to study different epochs of the universe, proposing observational strategies and analyzing detectability with future facilities.

Findings

Reionization-era HD signal detectable via cross-correlation with [CII] surveys.

Current projects are insufficient for HD detection during cosmic dawn.

HD intensity mapping during the dark ages is likely infeasible with existing or planned experiments.

Abstract

Hydrogen deuteride (HD) is prevalent in a wide variety of astrophysical environments, and measuring its large-scale distribution at different epochs can in principle provide information about the properties of these environments. In this paper, we explore the prospects for accessing this distribution using line intensity mapping of emission from the lowest rotational transition in HD, focusing on observations of the epoch of reionization ($z\sim6-10$) and earlier. We find the signal from the epoch of reionization to be strongest most promising, through cross-correlations within existing [CII] intensity mapping surveys. While the signal we predict is out of reach for current-generation projects, planned future improvements should be able to detect reionization-era HD without any additional observations, and would help to constrain the properties of the star-forming galaxies thought to play a key role in reionization. We also investigate several avenues for measuring HD during "cosmic dawn" ($z\sim10-30$), a period in which HD could provide one of the only complementary observables to 21$\,$cm intensity maps. We conclude that existing and planned facilities are poorly matched to the specifications desirable for a significant detection, though such a measurement may be achievable with sustained future effort. Finally, we explain why HD intensity mapping of the intergalactic medium during the cosmic dark ages ($z\gtrsim 30$) appears to be out of reach of any conceivable experiment.