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

TL;DR

This work analyzes the transverse proximity effect (TPE) around foreground quasars at $2\lesssim z\lesssim 3.5$ using over 10,000 quasar pairs from DESI Year 1 and the Lyα forest of background quasars. By measuring stacked Lyα transmission and inferring the optical depth ratio $\tau_{\rm prx}/\tau_{\rm IGM}$, the authors separate the impact of gas overdensity $\Delta$ from quasar ionization $\Gamma_q$, finding that environmental overdensities dominate the proximity signal and yielding $\Delta$ up to ~10 at $\sim 1\,h^{-1}{\rm Mpc}$. They compute $\Gamma_q$ from foreground luminosities, revealing that isotropic or narrowly beamed, time-variable emission can reconcile the lack of a clear luminosity dependence in the proximity profiles, despite expectations that brighter quasars produce stronger ionization. The study shows that the observed TPE is largely governed by dense environments rather than radiative clearing, with inferred overdensities that inform quasar halo environments and feedback, while providing constraints on quasar emission geometry and lifetimes. The results demonstrate DESI’s capacity to statistically map quasar environments via the Lyα forest and motivate forward radiative-transfer modeling to extract halo masses and radiation histories.

Abstract

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.

Figures (7)

• Figure 1: Spectrum of the Ly$\alpha$ forest of a background quasar that has a minimum projected separation of $1.86 \,h^{-1}{\rm Mpc}$ from a foreground quasar. The sky blue and orange circles indicate the spatial positions of the foreground and background quasars, respectively. The dashed vertical lines mark the Ly$\alpha$ wavelengths of these quasars. The pink-shaded region represents the proximity region, defined as a comoving line-of-sight separation of $\pm 50\,h^{-1}{\rm Mpc}$ relative to the foreground quasar. The foreground quasar is at $z=2.14$ (TargetID $= 39627480138519001$) and the background quasar is at $z=2.47$ (TargetID $= 39627480138519447$).
• Figure 2: Distributions and stacked Ly$\alpha$ transmissions for three subsamples divided by transverse distance (top), redshift (middle), and luminosity (bottom). Left: Histograms of each subsample, with solid vertical lines showing the median values. Right: Stacked Ly$\alpha$ transmissions around foreground quasars (solid) and corresponding control samples (dashed) as a function of line-of-sight separation from the foreground quasars (black dotted line). The pink dotted line marks the Si$\;$ absorption at $1206.5\text{\AA}$. Redshift and luminosity subsamples include all transverse separations $r_\perp = 0$–$2\,h^{-1}{\rm Mpc}$, and luminosity bins are selected to have similar redshift distributions.
• Figure 3: Ionization rates due to three foreground quasars at $z\simeq2.2$ (circle) on their background quasar sightlines (solid line), compared with the UVB ionization rate (green dotted line). Similar to Figure \ref{['fig:qso_pair_spectrum']}, but with circle size reflecting the luminosity of the foreground quasar: $\log(\nu L_\nu|_{1500\text{\AA}}/[\,{\rm erg\,s^{-1}}]) =$ 46.39 (black), 44.98 (orange), and 44.86 (sky blue).
• Figure 4: Luminosity dependence of Ly$\alpha$ optical depth variations in quasar proximity regions (solid lines). For comparison, we also show the expected contribution from quasar ionizing radiation alone (dashed lines), corresponding to the case where the overdensity is fixed at $\Delta=1$ in Equation \ref{['eq:tau_ratio']}. Line colors and vertical lines are the same as in Figure \ref{['fig:sub_all']}.
• Figure 5: Inferred gas overdensity profiles around quasars for the luminosity subsamples, shown as a function of line-of-sight separation $r_{\parallel}$ (left panel) and 3D distance $r_{\rm 3D}$ (right panel). The overdensities are derived from the $\tau_{\rm prx}/\tau_{\rm IGM}$ profiles in Figure \ref{['fig:tau_ratio_luminosity']} using Equation \ref{['eq:tau_ratio']}, with (solid lines) and without (dashed lines) including the contribution from quasar ionizing radiation. Line colors and vertical lines are the same as in Figure \ref{['fig:sub_all']}.