Baryon Acoustic Oscillations from the C IV Forest with DESI DR2

Abby Bault, Andrei Cuceu, Julien Guy, J. Aguilar, S. Ahlen, D. Bianchi, A. Brodzeller, D. Brooks, R. Canning, E. Chaussidon, T. Claybaugh, R. de Belsunce, A. de la Macorra, Arjun Dey, P. Doel, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, C. Gordon, D. Green, G. Gutierrez, C. Hahn, H. K. Herrera-Alcantar, K. Honscheid, M. Ishak, R. Joyce, S. Juneau, D. Kirkby, A. Kremin, C. Lamman, M. Landriau, L. Le Guillou, M. E. Levi, M. Manera, P. Martini, A. Meisner, R. Miquel, J. Moustakas, A. Muñoz-Gutiérrez, S. Nadathur, N. Palanque-Delabrouille, W. J. Percival, Matthew M. Pieri, C. Poppett, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, H. Seo, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver

First listed 2026-01-13 | Last updated 2026-01-13

Abstract

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 (ELG). We use quasars and ELGs from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) 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 CIV$\times$QSO cross-correlation. This translates into a 3.0% precision measurement of the ratio of the isotropic distance scale, $D_{\rm V}$, and the sound horizon at the drag epoch, $r_{\rm d}$, with $D_{\rm V}/r_{\rm d}(z_{\rm eff} = 1.92) = 30.3 \pm 0.9$. We make the first detection of the BAO feature in the CIV$\times$ELG cross-correlation at a significance of 2.5$σ$ and find $D_{\rm V}/r_{\rm d}(z_{\rm eff} = 1.47) = 24.6 \pm 1.0$.

Short digest

Using DESI DR2, the authors measure BAO by cross-correlating C IV forest absorption with the positions of 2.5M quasars and 3.1M ELGs, leveraging 1.5M high‑z quasar sightlines (z>1.3). They detect an isotropic BAO in C IV×QSO at 4.2σ, yielding DV/rd(z_eff=1.92)=30.3±0.9 (3.0%). They also make the first BAO detection in C IV×ELG at 2.5σ with DV/rd(z_eff=1.47)=24.6±1.0. These results establish the metal-line C IV forest as a viable high‑redshift BAO tracer that complements Lyα and extends DESI’s leverage on the expansion history.

Key figures to inspect

• Fig. 1 — Inspect the QSO and ELG redshift histograms to see the overlap that sets the effective redshifts for the BAO measurements; the hatched ELG subset indicates which galaxies actually contribute to the C IV cross-correlation.
• Fig. 2 — Use the example DESI quasar spectrum to verify the rest‑frame bands used: C IV window (1420–1520 Å) and Si IV window (1260–1375 Å); check how the SB2 (C IV) and SB1 (Si IV) regions avoid emission peaks and define the absorption fields.
• Fig. 3 — Examine the 2D correlation for C IV(SB2)×QSO and C IV(SB1)×QSO to locate the BAO half‑ring and assess anisotropy; note the oscillatory features attributed to quasar redshift errors in SB2 (cf. Section IV.2).
• Fig. 4 — Compare the 2D correlation for C IV×ELG with the QSO case; despite higher noise, look for the same half‑ring signature consistent with the reported 2.5σ BAO detection and how SB1 vs SB2 contribute.