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 |$\alpha$| 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 |$\alpha$| intensity mapping with broad-band optical images, by cross-correlating them with Ly |$\alpha$| forest data using a custom one-parameter estimator. We also develop an analytical large-scale Ly |$\alpha$| emission model with two parameters (average luminosity |$\langle L_{\rm Ly\alpha } \rangle$| and bias |$b_{\rm e}$|⁠) 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 |$\alpha$| forest data, setting guidelines for reducing images into Ly |$\alpha$| intensity maps. Given the transversal scales of our cross-correlation (26.4 arcmin, |$\sim$| 33 cMpc h−1), our study effectively integrates Ly |$\alpha$| emission over all the cosmic volume inside the DESI footprint at |$2.2 \lt z \lt 3.4$| (the g -band Ly |$\alpha$| redshift range). Over the parameter space (⁠|$\langle L_{\rm Ly\alpha } \rangle$|⁠ , |$b_{\rm e}$|⁠) sampled by our forecast, we find a 3 |$\sigma$| of large-scale structure in Ly |$\alpha$| 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 |$\alpha$| QSOs. Without a detection, we derive upper bounds on |$\langle L_{\rm Ly\alpha } \rangle$| competitive with optimistic literature estimates [ |$2.3 \pm 1 \times 10^{\rm 41}$| erg s−1 (cMpc |$^3$|⁠)−1 for DESI, and |$\sim$| 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 |$\alpha$| intensity mapping using next-generation imaging surveys is certain. Such detection would allow constraining |$\langle L_{\rm Ly\alpha } \rangle$|⁠ , and explore the constraining power of Ly |$\alpha$| intensity mapping as a cosmological probe. [ABSTRACT FROM AUTHOR]