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arXiv · 2507.21852

The Lyman-$α$ Forest from LBGs: First 3D Correlation Measurement with DESI and Prospects for Cosmology

Hiram K. Herrera-Alcantar·Eric Armengaud·Christophe Yèche·Calum Gordon·Laura Casas·Andreu Font-Ribera·Christophe Magneville·Corentin Ravoux·J. Aguilar·S. Ahlen·A. Anand·D. Brooks·E. Chaussidon·T. Claybaugh·A. Cuceu·K. S. Dawson·A. de la Macorra·Arjun Dey·P. Doel·S. Ferraro·J. E. Forero-Romero·E. Gaztañaga·S. Gontcho A Gontcho·A. X. Gonzalez-Morales·G. Gutierrez·J. Guy·C. Hahn·D. Kirkby·A. Kremin·O. Lahav·A. Lambert·M. Landriau·L. Le Guillou·M. Manera·P. Martini·A. Meisner·R. Miquel·A. Muñoz-Gutiérrez·S. Nadathur·N. Palanque-Delabrouille·W. J. Percival·F. Prada·I. Pérez-Ràfols·G. Rossi·E. Sanchez·D. Schlegel·M. Schubnell·J. Silber·D. Sprayberry·G. Tarlé·B. A. Weaver·R. Zhou·H. Zou
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Abstract

The Lyman-$α$ (Ly$α$) forest is a key tracer of large-scale structure at redshifts z > 2, traditionally studied using spectra of quasars. Here, we explore the viability Lyman Break Galaxies (LBGs) as alternative background sources for Ly$α$ forest studies. We analyze 4,151 Ly$α$ forest skewers extracted from LBG spectra obtained in the DESI pilot surveys in the COSMOS and XMM-LSS fields. We present the first measurement of the Ly$α$ forest auto-correlation function derived exclusively from LBG spectra, probing comoving separations up to 48 $h^{-1}$Mpc at an effective redshift of $z_\mathrm{eff}$ = 2.70. The measured signal is consistent with that from DESI DR2 quasar Ly$α$ forest spectra at a comparable redshift, validating LBGs as reliable background sources. We also measure the cross-correlation between the LBG Ly$α$ forest and 13,362 galaxy positions, showing that this observable serves as a sensitive diagnostic for galaxy redshift uncertainties and systematic offsets. Finally, using synthetic LBG spectra and Fisher forecasts, we show that a future wide-area survey over 5000 deg$^2$, targeting 1000 LBGs per deg$^2$ at similar signal-to-noise than our dataset, could enable Ly$α$ forest baryon acoustic oscillation (BAO) measurements with 0.4% precision on the isotropic BAO scale and 1.3% on the anisotropic (Alcock-Paczynski) scale. Combining BAO with a Ly$α$ forest full-shape analysis improves the AP constraint to 0.6%. These results open a new path for precision cosmology at high redshift using dense LBG samples.

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Hiram K. Herrera-Alcantar, Eric Armengaud, Christophe Yèche, Calum Gordon, Laura Casas, Andreu Font-Ribera, Christophe Magneville, Corentin Ravoux, J. Aguilar, S. Ahlen, A. Anand, D. Brooks, E. Chaussidon, T. Claybaugh, A. Cuceu, K. S. Dawson, A. de la Macorra, Arjun Dey, P. Doel, S. Ferraro, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, A. X. Gonzalez-Morales, G. Gutierrez, J. Guy, C. Hahn, D. Kirkby, A. Kremin, O. Lahav, A. Lambert, M. Landriau, L. Le Guillou, M. Manera, P. Martini, A. Meisner, R. Miquel, A. Muñoz-Gutiérrez, S. Nadathur, N. Palanque-Delabrouille, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, R. Zhou, H. Zou. 2025-12-29. The Lyman-$α$ Forest from LBGs: First 3D Correlation Measurement with DESI and Prospects for Cosmology. https://doi.org/10.1088/1475-7516%2F2025%2F12%2F053

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