Large-scale surveys of the quasar proximity effect

Rupert A. C. Croft, Patrick Shaw, Ann-Marsha Alexis, Nianyi Chen, Yihao Zhou, Tiziana Di Matteo, Simeon Bird, Patrick Lachance, Yueying Ni

First listed 2025-04-04 | Last updated 2025-08-21

Abstract

The UV radiation from high redshift quasars causes a local deficit in the neutral hydrogen absorption (Lyman-alpha forest) in their spectra, known as the proximity effect. Measurements from small samples of tens to hundreds of quasars have been used to constrain the global intensity of the UV background radiation, but so far the power of large-scale surveys such as the Sloan Digital Sky Survey and the Dark Energy Spectroscopic Instrument (DESI) survey has not been used to investigate the UV background in more detail. We develop a CDM-based halo model of the quasar proximity effect, which accounts by construction for the fact that quasars reside in overdense regions. We test this model on quasar Lyman-alpha spectra from the ASTRID cosmological hydrodynamic simulation, which includes self-consistent formation of quasar black holes and the intergalactic medium surrounding them. Fitting the model to individual quasar spectra, we constrain two parameters, r_eq (the radius at which the local quasar radiation intensity equals the background), and the quasar bias b_q (related to host halo mass). We find that r_eq can be recovered in an unbiased fashion with a statistical uncertainty of 25-50% from a single quasar spectrum. Applying such fitting to samples of millions of spectra from e.g., DESI would allow measurement of the UVBG intensity and its evolution with redshift with high precision. We use another, larger-scale, lower resolution simulation (Uchuu) to test how such a large sample of proximity effect measurements could be used to probe the spatial fluctuations in the intergalactic radiation field. We find that the large-scale structure of the UV radiation intensity could be mapped and its power spectrum measured on 100-1000 Mpc/h scales. This could allow the large-scale radiation field to join the density field as a dataset for constraining cosmology and the sources of radiation.

Short digest

Develops a CDM halo-model framework for the quasar proximity effect that explicitly includes overdense quasar environments and fits individual Lyα spectra for the radiation-equality radius r_eq and large-scale quasar bias b_q, validated on the ASTRID hydrodynamic simulation. Demonstrates that r_eq can be recovered without bias from a single spectrum with 25–50% statistical uncertainty. Scaling to DESI-sized samples enables precise measurements of the UV background intensity and its redshift evolution. Using the Uchuu simulation, shows that proximity-effect measurements could map spatial UV radiation fluctuations and measure their power spectrum on 100–1000 h−1 Mpc scales, making the radiation field a cosmological observable alongside density.

Key figures to inspect

• Figure 1: Compare real- vs redshift-space model curves with and without local quasar radiation and with the one-halo term removed to see how environment biases the transmitted-flux profile and where r_eq imprints a scale break.
• Figure 2: Slice through the Astrid volume centered on the brightest quasar (uniform UVBG only) to visualize the overdense IGM structure and neutral-fraction gradients before adding quasar radiation—use the slice thickness to gauge how these structures compare to expected r_eq scales.
• Figure 3: For the top-luminosity quasars, examine how r_eq scales with black-hole mass and how M_BH tracks halo mass; use the bottom panel’s trend versus halo mass to connect fitted b_q (or related clustering proxy) to host mass and identify scatter/outliers relevant for single-sightline fits.
• Figure 4: With a halo-mass–selected sample, check how r_eq and quasar properties populate the most massive halos and how the clustering/bias metric varies with M_h—highlighting selection effects that matter when aggregating proximity-effect measurements for large surveys.