Nathalie Palanque Delabrouille

We present a multi-redshift Baryon Acoustic Oscillations (BAO) analysis of the DESI Data Release 2 (DR2) Lyman- α (Ly α ) forest, splitting the forest auto-correlation and its cross-correlation with quasars into three redshift bins. We obtain BAO measurements at effective redshifts z _eff = 2.13, 2.40, and 2.81 with ∼ 2.0–2.5% precision per bin in the radial and transverse directions, corresponding to ∼ 1.1–1.2% precision for the isotropic BAO measurement. Using the same data products and modeling framework as the DESI DR2 Ly α BAO analysis, we validate the pipeline on 400 synthetic datasets and find unbiased BAO recovery with well-calibrated uncertainties. The measurements show an increase in the isotropic dilation parameter D _V / r _d from 30.26 ± 0.39 to 32.22 ± 0.47 and in the Alcock-Paczyński parameter D _M / D _H from 3.96 ± 0.15 to 5.63 ^+0.22 _-0.24 . The Hubble distance D _H / r _d decreases from 9.40 ± 0.20 to 7.22 ± 0.17, providing a direct measurement of the expansion history consistent with ΛCDM and the expected matter-dominated scaling, with H ( z ) ∝ (1 + z ) ⁿ giving n = 1.34 ± 0.16. The redshift split also provides a self-consistent measurement of clustering evolution: the Ly α forest bias evolves as (1 + z ) ^γ with γ _α = 3.05 ± 0.16, the RSD parameter has a redshift evolution described by γ _β = -0.97 ± 0.26, and the quasar bias evolves with γ _Q = 1.56 ± 0.23, consistent with independent quasar clustering measurements. Combining these three-bin BAO measurements with DESI DR2 galaxy and quasar BAO measurements yields cosmological constraints consistent with the single-bin Ly α BAO analysis in flat ΛCDM and w ₀ w _a CDM and improves curvature constraints by ∼ 12% in ΛCDM+Ω _K .

We present and validate a blinding method for the Lyman- α (Ly α ) forest analysis based on a modification of the Alcock-Paczynski test. In order to hide the background expansion history, the method employs a geometrical shift of each quasar (QSO) forest in wavelength space, once the quasar continuum has been fitted and the fluctuation field is extracted. The redshift positions for the QSO sample are also changed in a consistent manner. We show that the method remains effective when applied to real data, where contamination from metals and Lyman- β is intrinsically mixed with the Lyman- α forest. This limitation is primarily visible in the 1D correlation function, where other blinding strategies can mitigate the effect. To assess its effectiveness, the prescription is tested against a series of datasets of increasing complexity: from idealized low-noise mocks, to realistic DESI year one synthetic datasets, and finally to data from DESI first data release (DR1), using both the auto (Ly α ×Ly α ) and cross (Ly α × QSO) correlations. We find that the method robustly shifts the BAO peak position from the 3D correlation functions to the expected value for cosmology changes of around 5% in the matter content, without altering the shape of the posteriors in the model parameters. In conclusion, this catalog-level blinding strategy is a viable method for cosmological inference with the Lyman- α forest, particularly if a cross-analysis with other tracers using the same blinding strategy is pursued.

We present constraints on cosmic structure growth from the analysis of galaxy clustering and galaxy–galaxy lensing with galaxies from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. Our analysis includes four samples drawn from the Bright Galaxy Survey (BGS) and the luminous red galaxy (LRG) target classes. Projected galaxy clustering measurements from DESI are supplemented with lensing data from the Dark Energy Survey (DES), the Kilo-Degree Survey (KiDS), and the Hyper Suprime-Cam (HSC) survey around the same targets. Our method relies on a simulation-based modeling framework using the AbacusSummit simulations and a complex halo occupation distribution model that incorporates assembly bias. We analyze scales down to 0.4 h ⁻¹ Mpc for clustering and 2.5 h ⁻¹ Mpc for lensing, leading to stringent constraints on S 8 = σ 8 Ω m / 0.3 and Ω _m when fixing other cosmological parameters to those preferred by the cosmic microwave background. We find S 8 = 0.79 7 − 0.024 + 0.023 and Ω _m = 0.292 ± 0.011 when using lensing measurements from DES and KiDS. Similarly, for HSC, we find S 8 = 0.79 1 − 0.021 + 0.020 and Ω _m = 0.300 ± 0.009 when assuming the best-fit photometric redshift offset suggested by the HSC collaboration. Overall, our results are in good agreement with other results in the literature while continuing to highlight the constraining power of nonlinear scales.

Ly α emitters (LAEs) are star-forming galaxies with significant Ly α emission and are often used as tracers of large-scale structure at high redshift. We explore the relationship between the Ly α line profile and environmental density with spectroscopy from the Dark Energy Spectroscopic Instrument (DESI) of LAEs selected with narrowband photometry through the One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey. We use LAE surface density maps in the N419 ( z ∼ 2.45) and N501 ( z ∼ 3.12) narrow bands to probe the relationship between local environmental density and the Ly α line profile. In both narrow bands, we stack the LAE spectra in bins of environmental density and inside and outside of protocluster regions. The N501 data show ∼15% higher Ly α line luminosity for galaxies in protoclusters, suggesting increased star formation in these regions. However, the line luminosity is not appreciably greater in protocluster galaxies in the N419 band, suggesting a potential redshift evolution of this effect. The shape of the line profile itself does not vary with environmental density, suggesting that line shape changes are caused by local effects independent of a galaxy’s environment. These data indicate a potential relationship between LAE local environmental density, ionized gas distribution, and Ly α line luminosity.

We present details regarding a composite spectrum of quasar (QSO) absorption line systems. In this composite spectrum, we identify more than 70 absorption lines and observe oxygen and hydrogen emission features at a higher signal-to-noise ratio than any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 239,174 quasar spectra from the second data release of the Dark Energy Spectroscopic Instrument (DESI), each identified to have absorption lines resulting from such an interaction. By stacking these spectra in the rest frame of the absorption and calculating a median composite spectrum, we are able to isolate and enhance these absorption lines. We provide a full atlas of all detected absorption and emission lines as well as their fit centroids and equivalent width values. We additionally provide analysis of composites constructed using subsamples of absorbers in both redshift and Mg ii equivalent width bins. We find that there is minimal redshift evolution across the considered absorber samples. Additionally, we find that dense gas signatures, such as the presence of Na I absorption, are strongest in the highest equivalent width absorber sample, and that the strengths of the Si IV and C IV absorption lines show a relatively weak correlation with Mg II equivalent width. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers.

We present clustering measurements for high-redshift quasars using data from the Dark Energy Spectroscopic Instrument Data Release 2. Our sample consists of quasars with 2.0 < z < 3.5 in the luminosity range M ₁₄₅₀ ≤ -19.94 mag. We measure the mean quasar bias b _Q ( z ̅ = 2.48) = 3.61 ± 0.01 for the full sample of ∼ 715,000 quasars and quantify the redshift evolution of quasar bias by dividing the sample into four equal redshift bins. There is strong evolution of the quasar bias with redshift that is well fit by the function b _Q ( z ) = a [(1 + z ) ² - 6.565] + b with a = 0.230 ± 0.007 and b = 2.394 ± 0.035, and this fit is also a good match to lower redshift measurements in the literature. This bias evolution is consistent with a characteristic halo mass of M ̅ _h ∼ 10 ¹² M _⊙ that does not vary significantly with redshift. The inferred duty cycles for quasars in our sample are f _duty ∼ 10 ^-2 , staying mostly constant over redshifts. We investigate the luminosity dependence of quasar clustering by dividing each of our four redshift bins into three luminosity bins. The size of our quasar sample permits the first statistically significant measurement of the luminosity dependence of quasar bias at these redshifts. We measure weak dependence of quasar bias on luminosity at fixed redshift, inconsistent with no dependence, but weaker than predicted by a model in which quasar luminosity is tightly correlated with halo mass. These clustering measurements provide a stringent test for models of active black hole light curves and the black hole-halo connection at high redshift.

We present a measurement of baryon acoustic oscillations (BAO) in the cross correlation of triply ionized carbon (C iv ) absorption with the positions of quasars (QSO) and emission-line galaxies (ELGs). We use quasars and ELGs from the second data release (DR2) of the Dark Energy Spectroscopic Instrument survey. Our data sample consists of 2.5 million quasars, 3.1 million ELGs, and the C iv absorption is measured along the line of sight of 1.5 million high-redshift quasars with z > 1.3. We measure the isotropic BAO signal at 4.2 σ for the C IV ×QSO cross correlation. This translates into a 3.0% precision measurement of the ratio of the isotropic distance scale, D _V , and the sound horizon at the drag epoch, r _d , with D _V / r _d ( z _eff = 1.92) = 30.3 ± 0.9. We make the first detection of the BAO feature in the C IV ×ELG cross-correlation at a significance of 2.5 σ and find D _V / r _d ( z _eff = 1.47) = 24.6 ± 1.0.

We present a clustering analysis of Lyman- α emitters (LAEs) using spectroscopic observations from the Dark Energy Spectroscopic Instrument (DESI) of candidates selected from the Blanco/DECam Intermediate-Band Imaging Survey (IBIS). We measure the two-point correlation function and the power spectrum, including cross-correlations with DESI quasars. Using both analytical and halo occupation distribution (HOD) simulation-based modeling, we find a linear bias of b ∼ 2.31–2.62 for LAEs over the redshift range 2.26 < z < 3.41. The analytical modeling also provides constraints on the strength of radiative transfer effects, while the HOD analysis characterizes the LAE-halo connection across multiple models. Finally, we quantify the magnitude of non-perturbative clustering effects such as Fingers of God in the LAE population, providing essential input for the accurate modeling of LAE-based cosmological analyses in forthcoming high-redshift surveys such as DESI-II.

Quasar spectra carry the imprint of foreground intergalactic medium (IGM) through absorption features. In particular, absorption caused by neutral hydrogen gas, the “Lyα forest,” is a key spectroscopic tracer for cosmological analyses used to measure cosmic expansion and test physics beyond the standard model. Despite their importance, current methods for measuring Lyα absorption cannot directly derive the intrinsic quasar continuum and make strong assumptions on its shape, thus distorting the measured Lyα clustering. We present SpenderQ , a ML-based approach for directly reconstructing the intrinsic quasar spectra and measuring the Lyα forest from observations. SpenderQ uses the Spender spectrum autoencoder to learn a compact and redshift-invariant latent encoding of quasar spectra, combined with an iterative procedure to identify and mask absorption regions. To demonstrate its performance, we apply SpenderQ to 400,000 synthetic quasar spectra created to validate the Dark Energy Spectroscopic Instrument Year 1 Lyα cosmological analyses. SpenderQ accurately reconstructs the true intrinsic quasar spectra, including the broad Lyβ, Lyα, SiIV, CIV, and CIII emission lines. Redward of Lyα, SpenderQ provides percent-level reconstructions of the true quasar spectra. Blueward of Lyα, SpenderQ reconstructs the true spectra to < 5%. SpenderQ reproduces the shapes of individual quasar spectra more robustly than the current state-of-the-art. We, thus, expect it will significantly reduce biases in Lyα clustering measurements and enable studies of quasars and their physical properties. SpenderQ also provides informative latent variable encodings that can be used to, e.g., classify quasars with Broad Absorption Lines. Overall, SpenderQ provides a new data-driven approach for unbiased Lyα forest measurements in cosmological, quasar, and IGM studies.