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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

TL;DR

This paper delivers the most precise Baryon Acoustic Oscillation measurements from the Lyα forest to date using DESI DR2 data, exploiting a vastly enlarged quasar and Lyα sample and key methodological upgrades. Through an improved distortion matrix that includes redshift evolution and a refined metal-line modeling, the authors extract α∥ and α⊥ from auto- and cross-correlations, achieving 1.1% and 1.3% precision respectively and a 0.65% isotropic precision at z_eff=2.33. A new theoretical systematic accounts for potential Lyα BAO shifts due to non-linear growth, ensuring robust uncertainty propagation. The results are consistent with previous measurements and Planck cosmology, and the work lays groundwork for broader DR2 BAO and cosmological analyses, including lower-redshift galaxy/quasar BAO and future full-shape studies.

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.

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

TL;DR

This paper delivers the most precise Baryon Acoustic Oscillation measurements from the Lyα forest to date using DESI DR2 data, exploiting a vastly enlarged quasar and Lyα sample and key methodological upgrades. Through an improved distortion matrix that includes redshift evolution and a refined metal-line modeling, the authors extract α∥ and α⊥ from auto- and cross-correlations, achieving 1.1% and 1.3% precision respectively and a 0.65% isotropic precision at z_eff=2.33. A new theoretical systematic accounts for potential Lyα BAO shifts due to non-linear growth, ensuring robust uncertainty propagation. The results are consistent with previous measurements and Planck cosmology, and the work lays groundwork for broader DR2 BAO and cosmological analyses, including lower-redshift galaxy/quasar BAO and future full-shape studies.

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 . 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 and , where is the Hubble distance, is the transverse comoving distance, 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.

Paper Structure

This paper contains 23 sections, 17 equations, 13 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Spectroscopy
  4. Quasar Catalog
  5. Ancillary Catalogs
  6. Forest Measurements
  7. Correlations
  8. Distortion Matrix
  9. Modeling Metals
  10. Measurement
  11. Baseline Fit
  12. Goodness of fit
  13. Validation
  14. Validation with synthetic data
  15. Validation with blinded data
  16. ...and 8 more sections

Figures (13)

  • Figure 1: Left: Expected final DESI (dark blue outline) and the SDSS-DR16 footprint (red outline) together with the spatial distribution of DESI DR2 observed quasars (cyan). For reference we also show the Galactic plane (solid black line) and the Ecliptic plane (dotted black) line. Right: Number of quasars with different numbers of observations for the Ly$\alpha$ quasar sample in DESI DR2 (filled cyan) and DR1 (black line).
  • Figure 2: Quasar at $z = 3.20$ from the DESI DR2 dataset ($\mathrm{TargetID} = 39627696665266273$). The spectrum shows both regions where we measure the Ly$\alpha$ forest: Region A (indigo) extends from 1040 -- 1205 Å in the quasar rest frame and region B (purple) extends from 920 -- 1020 Å. We measure Ly$\alpha$ in both regions. The C IV and C III regions are highlighted in various shades of green. While there is almost no C III absorption, the C IV absorption spans leftward of the C IV doublet and thus is a contaminant in both region A and B of the Ly$\alpha$ forest. This is an atypically high SNR spectrum and we have smoothed it to better illustrate the quasar spectrum and the forest absorption. This quasar contains a DLA with $\mathrm{log_{10}\,N_{HI}} = 20.55$ at $z=2.93$.
  • Figure 3: Measured Ly$\alpha$(A)$\times$Ly$\alpha$(A) and Ly$\alpha$(A)$\times$Ly$\alpha$(B) auto-correlation functions (top and bottom panel). The different colors and markers correspond to different orientations with respect to the line-of-sight, with blue correlations being close to the line-of-sight $0.95<\mu<1$. The best fit model to all four correlations (see \ref{['sec:fit']}) is represented with solid curves. The dashed curves show the best fit model with additive broad-band corrections (see \ref{['sec:dataval']}).
  • Figure 4: Measured Ly$\alpha$(A)$\times$QSO and Ly$\alpha$(B)$\times$QSO cross-correlation functions (top and bottom panel). The different colors and markers correspond to different orientations with respect to the line-of-sight, with blue correlations being close to the line-of-sight $0.95<|\mu|<1$. The best fit model to all four correlations (see \ref{['sec:fit']}) is represented with solid curves. The dashed curves show the best fit model with additive broad-band corrections (see \ref{['sec:dataval']}).
  • Figure 5: Ly$\alpha$ BAO measurement of $\alpha_{\|}$ vs. $\alpha_{\perp}$ from DESI DR2 (red contour) compared to DESI DR1 (blue contour) and eBOSS DR16 (dashed, gray contours).
  • ...and 8 more figures