A New Boundary Condition on Reionization

The epoch of reionization (EoR) marks the last phase transition of hydrogen in our Universe, as it evolves from cold and neutral to hot and ionized in the intergalactic medium (IGM). While its endpoint and duration can be estimated from current observations, albeit with large uncertainties, there is no known avenue to constrain its onset. We propose a novel method based on the Pearson cross-correlation coefficient between 21-cm brightness temperature maps and line-intensity maps tracing star-formation (e.g., [OIII], CO, [CII]). This real-space estimator evolves from negative to positive as X-ray heating progresses, and saturates prior to the EoR. We predict a sharp drop from saturation during the earliest EoR stages, when the IGM ionized fraction is $x_{\rm HII}\lesssim 10\%$. We show that in standard scenarios, where IGM heating precedes reionization, the drop is a clear, model-robust signature that the EoR is still in its early stages, even when $\bar{x}_{\rm HII}$ cannot be measured precisely. This information is not accessible through the detection of an anticorrelation alone, which only indicates that reionization is ongoing. To assess the detectability of this feature, we provide a preliminary estimate of its signal-to-noise ratio in our fiducial scenario, assuming SPHEREx-like and SKAO-like noise levels, indicating that it is within reach of next-generation surveys. The detection of the Pearson drop therefore will provide a unique anchor for the EoR onset, and an upper bound on $x_{\rm HII}$, complementing existing probes and tightening constraints on early galaxy formation models.