Prospects for 21cm-Galaxy Cross-Correlations with HERA and the Roman High-Latitude Survey

TL;DR

This paper assesses the potential for detecting the cross-correlation between 21 cm signals and galaxy distributions during the Epoch of Reionization using HERA and Roman, forecasting a significant detection and discussing observational strategies.

Contribution

It presents the first detailed forecast for 21 cm-galaxy cross-correlation detection prospects with HERA and Roman, highlighting the importance of survey design and shot-noise limitations.

Findings

Forecasts a 14σ detection with overlapping 500 deg² surveys.

Shot-noise in galaxy counts limits detection sensitivity.

Deeper galaxy observations are more beneficial than wider coverage.

Abstract

The cross-correlation between the 21 cm field and the galaxy distribution is a potential probe of the Epoch of Reionization (EoR). The 21 cm signal traces neutral gas in the intergalactic medium and, on large spatial scales, this should be anti-correlated with the high-redshift galaxy distribution which partly sources and tracks the ionized gas. In the near future, interferometers such as the Hydrogen Epoch of Reionization Array (HERA) are projected to provide extremely sensitive measurements of the 21 cm power spectrum. At the same time, the Nancy Grace Roman Space Telescope (Roman) will produce the most extensive catalog to date of bright galaxies from the EoR. Using semi-numeric simulations of reionization, we explore the prospects for measuring the cross-power spectrum between the 21 cm and galaxy fields during the EoR. We forecast a 14$\sigma$ detection between HERA and Roman, assuming an overlapping survey area of 500 deg$^2$, redshift uncertainties of $\sigma_z = 0.01$ (as expected for the high-latitude spectroscopic survey of Ly$\alpha$-emitting galaxies), and an effective Ly$\alpha$ emitter duty cycle of $f_\mathrm{LAE} = 0.1$. Thus the HERA-Roman cross-power spectrum may be used to help verify 21 cm detections from HERA. We find that the shot-noise in the galaxy distribution is a limiting factor for detection, and so supplemental observations using Roman should prioritize deeper observations, rather than covering a wider field of view.