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

TL;DR

This work demonstrates the viability of using Ly$\alpha$ forest signals from Lyman Break Galaxy (LBG) spectra as an independent tracer of large-scale structure at $z>2$, presenting the first 3D Ly$\alpha$ auto-correlation from LBG forests and its cross-correlations with LBGs and LAEs. By adapting the DESI Ly$\alpha$ pipeline to low-SNR LBG data, the authors show that LBG forests can reproduce quasar-based Ly$\alpha$ results at similar redshifts and provide a dense tracer set for cosmology. They model contaminants (HCDs, metals) and correlated noise, quantify redshift-offset corrections, and assess how sample purity affects inferred parameters. Forecasts indicate that a future wide-area (5,000 deg$^2$) survey with ~1000 LBGs deg$^{-2}$ could yield sub-percent isotropic BAO precision and percent-level Alcock–Paczynski constraints, especially when combining auto- and cross-correlation measurements and possibly full-shape analyses. Overall, LBG-based Ly$\alpha$ forest studies offer a promising avenue for high-redshift precision cosmology, complementary to quasar-based investigations and expanding the redshift range for BAO and AP tests.

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.

Figures (10)

• Figure 1: Angular positions of the $\textrm{CL}>0.995$ LBG sample in the COSMOS (left) and XMM (right) fields used for our analysis. The COSMOS footprint covers approximately $6~\deg^2$, while the XMM footprint spans about $5~\deg^2$.
• Figure 2: Filled histogram: $r$-band magnitude distribution of LBGs with $\mathrm{CL}>0.995$ in the COSMOS and XMM fields, whose Ly$\alpha$ forests pass our quality cuts. The mean forest SNR as a function of $r$-band magnitude is shown with purple points; error bars represent the standard error of the mean. The effective exposure times for these observations range from 2 to 5 hours, with a median value of 4.5 hours. A small number of bright LBGs have longer exposures due to repeated observations.
• Figure 3: Mean rest-frame continuum in the Ly$\alpha$ region as measured from LBG spectra on the COSMOS (blue line) and XMM (red line) fields. The green shaded vertical lines indicate known spectral line features caused by stellar and interstellar absorption. By construction from the continuum fitting procedure, $\overline{C}_{\rm LBG}$ is arbitrarily normalized.
• Figure 4: Ly$\alpha\times$Ly$\alpha$ auto‑correlation measured from LBG spectra with $\mathrm{CL}>0.995$ in COSMOS (left), XMM (center), and the combined fields (right), shown over a $(r_\parallel,r_\perp)$ grid. While some visual differences are apparent between COSMOS and XMM, we have verified that the measurements are statistically consistent; these differences are likely due to noise and statistical fluctuations.
• Figure 5: Ly$\alpha\times$Ly$\alpha$ auto-correlation from the COSMOS+XMM LBG forest combined sample as a function of $r_\parallel$ (left) and $r_\perp$ (right), averaged over the $1-6\ h^{-1}\,\mathrm{Mpc}$ range of the complementary coordinate. The black line shows the results from the DESI.DR2.BAO.lya quasar forest sample where forest pairs have been restricted to match the effective redshift of our LBG forest signal. The (almost indistinguishable) shaded black region corresponds to the 1$\sigma$ uncertainty.