Stars and reionization: The cross-correlation of the 21 cm line and the near-infrared background

With improving telescopes, it may now be possible to observe the Epoch of Reionization in multiple ways. We examine two of these observables-the excess light in the near-infrared background that may be due to high-redshift stars and ionized HII bubbles, and the 21 cm emission from neutral hydrogen. Because these two forms of emission should result from different,mutually exclusive regions, an anticorrelation should exist between them.We discuss the strengths of using cross-correlations between these observations to learn more about highredshift star formation and reionization history. In particular, we create simulated maps of emission from both the near-infrared background and 21 cm emission. We find that these observations are anticorrelated, with the strongest anticorrelation originating from times when the universe is half ionized. This result is robust and does not depend on the properties of the stars themselves. Rather, it depends on the ionization history. Cross-correlations can provide redshift information, which the near-infrared background cannot provide alone. In addition, cross-correlations can help separate foreground emission from the true high-redshift component, making it possible to say with greater certainty that we are indeed witnessing the Epoch of Reionization.