Reionization Science from Cross-Correlations with the Roman Space Telescope

PI: Lidz, Adam, University Of Pennsylvania
Wide-Field Science – Regular

The Roman Space Telescope’s planned high latitude survey will enable measurements of millions of reionization-era galaxies (at z > 6) across wide regions of the sky (~2,000 square degrees). These unprecedented observations will enable cross-correlations with other tracers of the Epoch of Reionization (EoR), including measurements of the redshifted 21 cm line from HERA, intensity maps of the extragalactic background light (EBL) from the SPHEREx mission, and observations of the patchy kinetic Sunyaev-Zel'dovich (kSZ) effect from upcoming cosmic microwave background (CMB) surveys. An important advantage of the cross-correlation measurements is that (unshared) foreground contamination does not produce an average bias, unlike in an auto-power spectrum where residual foregrounds may masquerade as signal. As such, cross-correlations with the redshifted 21 cm/intensity mapping surveys may provide smoking-gun verifications of putative 21 cm and reionization-era EBL detections. Likewise, the Roman galaxy-kSZ cross-correlation can help separate the reionization-era patchy kSZ signal from other contributions to the CMB anisotropies. Furthermore, these cross-correlations will aid in extracting valuable information from the upcoming data regarding the timing of reionization and the size distribution of ionized bubbles at different stages of the reionization process.

In order to best prepare for the exciting prospects of reionization-era cross-correlations with Roman galaxies, we propose to: 1) further develop models for the abundance and clustering of the galaxies observable by Roman. This will include a detailed treatment of Lyman-alpha scattering from surrounding neutral hydrogen, which will modulate the observable abundance of Lyman-alpha emitting galaxies. We will model the impact of this scattering on the auto-power spectrum of Lyman-alpha emitting galaxies and their cross-correlations with 21 cm measurements. 2) Quantify the prospects for cross-correlating all-sky SPHEREx EBL measurements and CMB observations of the patchy kSZ signal with Lyman-alpha emitters and Lyman-break galaxies from Roman. 3) For the first time, we will explore how Roman measurements of the post-reionization galaxy distribution can be used to construct optimal filters to remove post-reionization contributions in both SPHEREx EBL maps and CMB estimates of the kSZ signal. Since the post-reionization signals add dominant noise contributions to reionization-era cross-correlations involving the EBL and kSZ signals, these filtering steps may boost the expected signal-to-noise ratio of the cross-correlations significantly. 4) Start to quantify the benefits of the joint cross-correlation analyses for inferring reionization physics parameters. 5) Use insights from the first four projects to help optimize the Roman survey strategy for carrying out cross-correlations with redshifted 21 cm, SPHEREx EBL measurements, and kSZ observations. Taken together, these efforts will significantly advance the science reach of future NASA missions, including Roman and SPHEREx, while also aiding ground-based 21 cm and CMB surveys.