DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest

DESI Collaboration, M. Abdul-Karim, J. Aguilar, S. Ahlen, C. Allende Prieto, O. Alves, A. Anand, U. Andrade, E. Armengaud, A. Aviles, S. Bailey, A. Bault, J. Behera, S. BenZvi, D. Bianchi, C. Blake, A. Brodzeller, D. Brooks, E. Buckley-Geer, E. Burtin, R. Calderon, R. Canning, A. Carnero Rosell, P. Carrilho, L. Casas, F. J. Castander, R. Cereskaite, M. Charles, E. Chaussidon, J. Chaves-Montero, D. Chebat, T. Claybaugh, S. Cole, A. P. Cooper, A. Cuceu, K. S. Dawson, R. de Belsunce, A. de la Macorra, A. de Mattia, N. Deiosso, J. Della Costa, A. Dey, B. Dey, Z. Ding, P. Doel, J. Edelstein, D. J. Eisenstein, W. Elbers, P. Fagrelius, K. Fanning, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, C. Garcia-Quintero, L. H. Garrison, E. Gaztañaga, H. Gil-Marín, S. Gontcho A Gontcho, A. X. Gonzalez-Morales, C. Gordon, D. Green, G. Gutierrez, J. Guy, C. Hahn, M. Herbold, H. K. Herrera-Alcantar, M. -F. Ho, K. Honscheid, C. Howlett, D. Huterer, M. Ishak, S. Juneau, N. G. Karaçaylı, R. Kehoe, S. Kent, D. Kirkby, T. Kisner, F. -S. Kitaura, S. E. Koposov, A. Kremin, O. Lahav, C. Lamman, M. Landriau, D. Lang, J. Lasker, J. M. Le Goff, L. Le Guillou, A. Leauthaud, M. E. Levi, Q. Li, T. S. Li, K. Lodha, M. Lokken, C. Magneville, M. Manera, P. Martini, W. L. Matthewson, P. McDonald, A. Meisner, J. Mena-Fernández, R. Miquel, J. Moustakas, D. Muñoz Santos, A. Muñoz-Gutiérrez, A. D. Myers, J. A. Newman, G. Niz, H. E. Noriega, E. Paillas, N. Palanque-Delabrouille, J. Pan, W. J. Percival, I. Pérez-Ràfols, M. M. Pieri, C. Poppett, F. Prada, D. Rabinowitz, A. Raichoor, C. Ramírez-Pérez, M. Rashkovetskyi, C. Ravoux, J. Rich, C. Rockosi, A. J. Ross, G. Rossi, V. Ruhlmann-Kleider, E. Sanchez, N. Sanders, S. Satyavolu, D. Schlegel, M. Schubnell, H. Seo, A. Shafieloo, R. Sharples, J. Silber, F. Sinigaglia, D. Sprayberry, T. Tan, G. Tarlé, P. Taylor, W. Turner, F. Valdes, M. Vargas-Magaña, M. Walther, B. A. Weaver, M. Wolfson, C. Yèche, P. Zarrouk, R. Zhou, H. Zou

First listed 2025-03-18 | Last updated 2025-06-30

Abstract

We present the Baryon Acoustic Oscillation (BAO) measurements with the Lyman-alpha (LyA) forest from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) survey. Our BAO measurements include both the auto-correlation of the LyA forest absorption observed in the spectra of high-redshift quasars and the cross-correlation of the absorption with the quasar positions. The total sample size is approximately a factor of two larger than the DR1 dataset, with forest measurements in over 820,000 quasar spectra and the positions of over 1.2 million quasars. We describe several significant improvements to our analysis in this paper, and two supporting papers describe improvements to the synthetic datasets that we use for validation and how we identify damped LyA absorbers. Our main result is that we have measured the BAO scale with a statistical precision of 1.1% along and 1.3% transverse to the line of sight, for a combined precision of 0.65% on the isotropic BAO scale at $z_{eff} = 2.33$. This excellent precision, combined with recent theoretical studies of the BAO shift due to nonlinear growth, motivated us to include a systematic error term in LyA BAO analysis for the first time. We measure the ratios $D_H(z_{eff})/r_d = 8.632 \pm 0.098 \pm 0.026$ and $D_M(z_{eff})/r_d = 38.99 \pm 0.52 \pm 0.12$, where $D_H = c/H(z)$ is the Hubble distance, $D_M$ is the transverse comoving distance, $r_d$ is the sound horizon at the drag epoch, and we quote both the statistical and the theoretical systematic uncertainty. The companion paper presents the BAO measurements at lower redshifts from the same dataset and the cosmological interpretation.

Short digest

DESI DR2 doubles the Lyα-forest dataset over DR1, analyzing both the Lyα auto-correlation and the Lyα–QSO cross-correlation from >820k quasar spectra and >1.2M quasar positions. It delivers sub‑percent BAO distances at zeff=2.33: 1.1% radial and 1.3% transverse, for 0.65% isotropic precision, measuring DH/rd=8.632±0.098(stat)±0.026(sys) and DM/rd=38.99±0.52(stat)±0.12(sys). The team details analysis upgrades, validation on improved synthetic data, and refined DLA identification. Motivated by nonlinear BAO shift studies, they include—for the first time in Lyα BAO—a dedicated systematic error term, strengthening high‑z expansion constraints.

Key figures to inspect

• Figure 1 — Footprint and target statistics: compare the DESI DR2 QSO sky coverage against SDSS-DR16 and inspect the right-panel histogram of repeat observations to gauge DR2 depth and cadence relative to DR1 for Lyα S/N.
• Figure 2 — Example DR2 quasar spectrum: locate Lyα forest regions A (1040–1205 Å rest) and B (920–1020 Å), note the placement of C IV/C III metal contamination, and see how a DLA in the sightline is identified/treated for the analysis.
• Figure 3 — Lyα auto-correlation (regions A and B): examine the BAO peak across line-of-sight orientation bins (μ; blue near-parallel) and compare solid (fiducial) vs dashed (with broadband terms) fits to see how anisotropic information constrains radial vs transverse scales.
• Figure 4 — Lyα–QSO cross-correlation: check the BAO signal’s consistency with the auto case across μ bins and regions A/B, and how the combined modeling tightens the distance constraints while controlling broadband systematics.