Measurements of quasar proximity zones with the Lyman-$α$ forest of DESI Y1 quasars

The intergalactic medium (IGM) around quasars is shaped by their dense environments and by their excess ionizing radiation, forming a “quasar proximity zone” whose size and anisotropy depend on the quasar’s halo mass, luminosity, age, and radiation geometry. Using over 10,000 quasar pairs from the Dark Energy Spectroscopic Instrument (DESI) Year 1 data, with projected comoving separations $r_{\perp} < 2,h^{-1}{\rm Mpc}$, we investigate how the proximity zone of foreground quasars at $z\sim2{\rm-}3.5$ affects Lyman-alpha absorption in their background quasars. The large DESI sample enables unprecedented precision in measuring this “transverse proximity” effect, allowing a detailed investigation of the signal’s dependence on the projected separation of quasar pairs and the luminosity of the foreground quasar. We find that enhanced gas clustering near quasars dominates over their ionizing effect, leading to stronger absorption on neighboring sightlines. Under the assumption that quasar ionizing luminosity is isotropic and steady, we infer the IGM overdensity profile in the vicinity of quasars, finding overdensities as high as $Δ\sim 10$ at comoving distance $\sim 1,h^{-1}{\rm Mpc}$ from the most luminous systems. Surprisingly, however, we find no significant dependence of the proximity profile on the luminosity of the foreground quasar. This lack of luminosity dependence could reflect a cancellation between higher ionizing flux and higher gas overdensity, or it could indicate that quasar emission is highly time variable or anisotropic, so that the observed luminosity does not trace the ionizing flux on nearby sightlines.

💡 Research Summary

This paper presents a comprehensive analysis of the transverse proximity effect (TPE) using the unprecedentedly large sample of quasar pairs from the Dark Energy Spectroscopic Instrument (DESI) Year‑1 (Y1) data release. The authors identify more than 10,000 foreground–background quasar pairs with projected comoving separations r⊥ < 2 h⁻¹ Mpc and foreground redshifts 2 ≲ z ≲ 3.5. By stacking the Ly α forest transmission in the background spectra and sorting the pairs by transverse distance, redshift, and foreground luminosity, they obtain high‑precision measurements of how the foreground quasar influences the intergalactic medium (IGM) along neighboring sightlines.

Contrary to the naive expectation that a luminous quasar should ionize the surrounding hydrogen and thus reduce Ly α absorption, the DESI data reveal a net increase in absorption near the foreground quasars. The authors interpret this as a dominance of gas overdensity (clustering) over the ionizing radiation field. Using a simple model that assumes isotropic, steady ionizing output, they decompose the observed optical depth into contributions from the background UVB, the quasar’s ionizing flux, and a density enhancement factor Δ(r). Fitting the stacked transmission yields an overdensity profile that reaches Δ ≈ 10 at a comoving distance of ∼1 h⁻¹ Mpc from the most luminous quasars, indicating that these objects reside in highly biased regions that will likely evolve into massive galaxy clusters.

Surprisingly, the strength of the TPE shows no statistically significant dependence on the foreground quasar’s UV luminosity. The authors discuss two plausible explanations. First, brighter quasars may indeed emit more ionizing photons, but they also tend to inhabit denser environments; the increased ionization and the stronger density‑driven absorption can cancel each other, masking any luminosity trend. Second, the lack of correlation may point to substantial temporal variability or anisotropy in quasar emission. If the ionizing output fluctuates on timescales shorter than the light‑travel time across ∼1 Mpc (∼3 Myr), or if the radiation is beamed away from the transverse direction, the observed luminosity would not faithfully trace the ionizing flux experienced by the background sightline.

The paper places these findings in the context of earlier work. Small‑scale studies such as the Quasars Probing Quasars (QPQ) survey reported either enhanced absorption or a null TPE signal, but suffered from limited pair numbers and large statistical uncertainties. The DESI results, with an order‑of‑magnitude larger sample, demonstrate that the TPE signal is robustly measurable and that gas clustering must be accounted for when interpreting Ly α forest–quasar cross‑correlations. Moreover, the inferred overdensity profile provides an independent probe of quasar host‑halo masses, complementing clustering analyses.

Methodologically, the authors describe the DESI instrumentation, data reduction pipeline, and quasar catalog construction, emphasizing the high spectral resolution (R ≈ 2000–5000) and wide wavelength coverage (3600–9800 Å) that enable reliable Ly α forest extraction. They construct control samples matched in redshift and signal‑to‑noise to isolate the TPE from generic IGM fluctuations. The modeling framework incorporates the known UV background intensity, the quasar’s ionizing photon rate (derived from its observed LUV under the isotropy assumption), and a parametric form for Δ(r). Sensitivity tests show that reasonable variations in the UVB or quasar age do not alter the main conclusion that overdensity dominates the signal.

Implications of the work are twofold. First, the absence of a clear luminosity dependence suggests that quasar lifetimes may be shorter than a few Myr, or that emission is highly directional, challenging simple models of steady, isotropic quasar activity. Second, the strong overdensities imply that quasars at z ≈ 2–3 are excellent tracers of the most massive dark‑matter halos, offering a pathway to map the progenitors of present‑day galaxy clusters.

Finally, the authors highlight the future potential of TPE studies with DESI and upcoming surveys. As DESI continues to collect data, the number of close quasar pairs will increase dramatically, allowing finer binning in redshift, luminosity, and separation, and enabling similar analyses at higher redshifts (z > 5) where the IGM is more neutral. Such work will open a new window on “light‑echo” tomography, providing direct constraints on quasar emission geometry, variability, and the timing of cosmic reionization.