THE GALAXY–IGM CONNECTION IN THESAN: OBSERVABILITY AND INFORMATION CONTENT OF THE GALAXY–LYMAN-α CROSS-CORRELATION AT Z≥6

Abstract

The galaxy–Lyman-α cross-correlation (GaLαCC) is a promising tool to study the interplay of galaxies and inter-galactic medium (IGM) in the first billion years of the Universe. Here we thoroughly characterise the impact of observational limitations on our ability to retrieve the intrinsic GaLαCC and provide new physical insights on its origin and connection to other IGM properties. This is extremely relevant to identify promising datasets, design future surveys and assess the limitations of current measurements. We find that sightline-to-sightline variations demand at least 25 independent sightlines to quantitatively recover the true signal. Once this condition is met, the intrinsic signal can be recovered even for a relatively low signal-to-noise ratio and spectral resolution. The galaxy selection method does not affect the inferred GaLαCC and lightcone effects are relevant whenever observations span a redshift window broader than Δz ≳ 0.4. We discuss the implication for previous theoretical studies that did not account for them. We elucidate explicitly for the first time the physical origin of the GaLαCC and demonstrate that this signal is collectively sourced by the ensemble of galaxies residing in overdense regions rather than individual objects. We show that the GaLαCC measured for opaque sightlines shows a larger peak at smaller scales with respect to transparent lines of sight. We connect this to the evolution of the mean free path of ionizing photons, showing that the GaLαCC peak position has a very similar evolution but on smaller scales, as it probes only the core of ionized regions. Finally, we discuss which ongoing surveys can be used to measure the GaLαCC and provide an initial analysis of future developments, including using galaxies as background sources, and the application to helium reionization. Our results outline a bright future for the GaLαCC as a tool to unveil the galaxy–IGM interplay during the first billion years of the Universe.