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Being the most prominent HI 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 L ₋ₘ and bias b ₄) that respects observational constraints from QSO luminosity functions. We compute a forecast for DECaLS/BASS g-band images cross-correlated with 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), our study effectively integrates Ly emission over all the cosmic volume inside the DESI footprint at 2. 2 < z < 3. 4 (the g-band Ly redshift range). Over the parameter space (L ₋ₘ, b ₄) sampled by our forecast, we find a 3 of large-scale structure in Ly likely, with a probability of detection of 23. 95\% for DESI-DECaLS/BASS, and 54. 93\% for a hypothetical DESI phase II with twice as much Ly QSOs. Without a detection, we derive upper bounds on L ₋ₘ competitive with optimistic literature estimates (2. 3 1 10^ 41 erg/s/cMpc³ for DESI, and 35\% 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. abridged
Renard et al. (Wed,) studied this question.
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