Measuring the ultraviolet background at z≃3 from the quasar proximity effect in DESI DR1
Abstract
Ionizing radiation from quasars reduces the abundance of neutral hydrogen in the surrounding intergalactic medium (IGM), suppressing Lyα absorption. This absorption deficit, known as the quasar proximity effect, can be used to measure the ultraviolet background (UVBG). We measure the hydrogen photoionization rate of the UVBG in Data Release 1 of the Dark Energy Spectroscopic Instrument (DESI). Our sample contains 6,245 quasars over 2.75≤ z<3.25, selected with rest-frame absolute magnitudes at 1450 Angstroms of M1450≤−25.5 and signal-to-noise ratios of at least 3 measured redward of Lyα. We reconstruct quasar continua with principal component analysis trained on Hubble Space Telescope spectra and stack the resulting transmitted-flux profiles. We fit the resulting stacked profile with a halo model that accounts for reduced absorption from quasar radiation and enhanced absorption in the surrounding overdense environment. We constrain the environmental contribution using independent measurements of quasar clustering bias, while the transmission profile constrains the UVBG. To validate our method, we construct mock spectra matched to the luminosity and noise properties of our DESI sample using the Astrid hydrodynamical simulation at z=3. We apply the same analysis pipeline to the mock spectra and DESI data, testing the recovery of known input photoionization rates and using the results to calibrate the final UVBG measurement. We obtain ΓHI=(1.376+0.095−0.076)×10−12 s−1 at z≃3. The statistical errors are obtained by bootstrap resampling the DESI sightlines. We also estimate a systematic uncertainty of +0.837−0.521×10−12 s−1.
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BibTeX
@misc{shaw2026measuring,
title = {{Measuring the ultraviolet background at $z\simeq3$ from the quasar proximity effect in DESI DR1}},
author = {Patrick Shaw and Rupert A. C. Croft},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.04316},
eprint = {2611.04316},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.04316},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Measuring the ultraviolet background at z≃3 from the quasar proximity effect in DESI DR1
AU - Patrick Shaw
AU - Rupert A. C. Croft
PY - 2026
DA - 2026/11/03
PB - arXiv
DO - 10.48550/arxiv.2611.04316
UR - https://arxiv.org/abs/2611.04316
AB - Ionizing radiation from quasars reduces the abundance of neutral hydrogen in the surrounding intergalactic medium (IGM), suppressing Lyα absorption. This absorption deficit, known as the quasar proximity effect, can be used to measure the ultraviolet background (UVBG). We measure the hydrogen photoionization rate of the UVBG in Data Release 1 of the Dark Energy Spectroscopic Instrument (DESI). Our sample contains 6,245 quasars over 2.75≤ z<3.25, selected with rest-frame absolute magnitudes at 1450 Angstroms of M_1450≤−25.5 and signal-to-noise ratios of at least 3 measured redward of Lyα. We reconstruct quasar continua with principal component analysis trained on Hubble Space Telescope spectra and stack the resulting transmitted-flux profiles. We fit the resulting stacked profile with a halo model that accounts for reduced absorption from quasar radiation and enhanced absorption in the surrounding overdense environment. We constrain the environmental contribution using independent measurements of quasar clustering bias, while the transmission profile constrains the UVBG. To validate our method, we construct mock spectra matched to the luminosity and noise properties of our DESI sample using the Astrid hydrodynamical simulation at z=3. We apply the same analysis pipeline to the mock spectra and DESI data, testing the recovery of known input photoionization rates and using the results to calibrate the final UVBG measurement. We obtain Γ_HI=(1.376^+0.095_−0.076)×10^−12 s^−1 at z≃3. The statistical errors are obtained by bootstrap resampling the DESI sightlines. We also estimate a systematic uncertainty of ^+0.837_−0.521×10^−12 s^−1.
ER -