Probing the cosmic web in Lyα emission over large scales: An intensity mapping forecast for DECaLS/BASS and DESI

Being the most prominent H i line, Ly permeates the cosmic web in emission. Despite its potential as a cosmological probe, its detection on large scales remains elusive. We present a new methodology to perform Ly intensity mapping with broad-band optical images, by cross-correlating them with Ly forest data using a custom one-parameter estimator. We also develop an analytical large-scale Ly emission model with two parameters (average luminosity and bias) that respects observational constraints from quasar (QSO) luminosity functions. We compute a forecast for Dark Energy Camera Legacy Survey (DECaLS)/Beijing-Arizona Sky Survey (BASS) g-band images cross-correlated with Dark Energy Spectroscopic Instrument (DESI) Ly forest data, setting guidelines for reducing images into Ly intensity maps. Given the transversal scales of our cross-correlation (26.4 arcmin, 33 cMpc h-1), our study effectively integrates Ly emission over all the cosmic volume inside the DESI footprint at (the g-band Ly redshift range). Over the parameter space (,) sampled by our forecast, we find a 3 of large-scale structure in Ly likely, with a probability of detection of 23.95 per cent for DESI-DECaLS/BASS, and 54.93 per cent for a hypothetical DESI phase-II with twice as much Ly QSOs. Without a detection, we derive upper bounds on competitive with optimistic literature estimates [erg s-1 (cMpc)-1 for DESI, and 35 per cent lower for its hypothetical phase-II]. Extrapolation to the DESI-Rubin overlap shows that a detection of large-scale structure with Ly intensity mapping using next-generation imaging surveys is certain. Such detection would allow constraining, and explore the constraining power of Ly intensity mapping as a cosmological probe.