The spin-temperature dependence of the 21cm -- LAE cross-correlation

Abstract

Cross-correlating 21cm with known cosmic signals will be invaluable proof of the cosmic origin of the first 21cm detections. As some of the widest fields available, comprising thousands of sources with reasonably known redshifts, narrow-band Lyman alpha emitter (LAE) surveys are an obvious choice for such cross-correlation. Here we revisit the 21cm -- LAE cross-correlation, relaxing the common assumption of reionization occurring in a pre-heated intergalactic medium (IGM). Using specifications from the Square Kilometre Array and the Subary Hyper Supreme-Cam, we present new forecasts of the 21cm -- LAE cross-correlation function at $z\sim 7$. We sample a broad parameter space of the mean IGM neutral fraction and spin temperature, (${\bar{x}}_{\mathrm{HI}}$, ${\bar{T}}_{\mathrm{S}}$). The sign of the cross-correlation roughly follows the sign of the 21cm signal: ionized regions which surround LAEs correspond to relative hot spots in the 21cm signal when the neutral IGM is colder than the CMB, and relative cold spots when the neutral IGM is hotter than the CMB. The amplitude of the cross-correlation function generally increases with increasing ${\bar{x}}_{\mathrm{HI}}$, following the increasing bias of the cosmic HII regions. As is the case for 21cm, the strongest cross signal occurs when the IGM is colder than the CMB, providing a large contrast between the neutral regions and the ionized regions which host LAEs. We also vary the topology of reionization and the epoch of X-ray heating. The cross-correlation during the first half of reionization is sensitive to these topologies, and could thus be used to constrain them.

9 pages, 3 figures

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• The 21cm-galaxy cross-correlation: Realistic forecast for 21cm signal detection and reionisation constraints