🔭 astrophysics

Measurements of quasar proximity zones with the Ly $\alpha$ forest of DESI Y1 quasars

Using over 10,000 quasar pairs from DESI Year 1 data, this study reveals that enhanced gas clustering dominates the transverse proximity effect around quasars at $z\sim2\text{--}3.5$ , leading to stronger Ly $\alpha$ absorption on neighboring sightlines while surprisingly showing no significant dependence on foreground quasar luminosity, which suggests either a cancellation between ionizing flux and overdensity or highly time-variable and anisotropic quasar emission.

Original authors: Ryuichiro Hada, Paul Martini, David H. Weinberg, Zheng Zheng, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, E.

Published 2026-05-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ryuichiro Hada, Paul Martini, David H. Weinberg, Zheng Zheng, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, E. Gaztañaga, G. Gutierrez, J. Guy, H. K. Herrera-Alcantar, K. Honscheid, M. Ishak, R. Joyce, D. Kirkby, T. Kisner, A. Kremin, C. Lamman, M. Landriau, L. Le Guillou, A. Meisner, R. Miquel, A. Muñoz-Gutiérrez, N. Palanque-Delabrouille, W. J. Percival, C. Poppett, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, H. Zou

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a vast, foggy ocean. Most of this ocean is made of a thin, invisible gas called the "Intergalactic Medium" (IGM). Occasionally, this fog gets thick enough to block light, creating what astronomers call the "Lyman-alpha forest"—a series of dark lines in the light from distant objects, like trees in a forest blocking your view.

Now, imagine a quasar as a massive, blindingly bright lighthouse sitting in the middle of this foggy ocean. This lighthouse is powered by a supermassive black hole. The light from this lighthouse is so intense that it should theoretically "burn away" the fog around it, creating a clear, transparent bubble known as a proximity zone.

This paper is like a massive survey of thousands of these lighthouses and the fog around them, using data from the DESI (Dark Energy Spectroscopic Instrument) telescope. Here is what the researchers found, explained simply:

1. The Setup: A Game of "Look Behind the Lighthouse"

The researchers didn't just look at the light from the lighthouse itself. Instead, they looked for pairs of quasars. They found a "foreground" lighthouse (the one closer to us) and a "background" lighthouse (one even further away) that happened to be lined up almost perfectly behind the first one.

By looking at the light from the background lighthouse as it passed near the foreground one, they could see how the foreground lighthouse affected the fog in between. It's like shining a flashlight through a window to see how a bright lamp sitting on the windowsill changes the air right next to it.

They analyzed over 10,000 of these pairs, a number much larger than any previous study, giving them a very clear picture.

2. The Big Surprise: The Fog is Thicker, Not Clearer

The scientists expected to see the "fog" (the gas) become thinner and clearer near the lighthouse because the intense radiation should ionize (strip electrons from) the gas, making it transparent.

Instead, they found the opposite.
The fog was actually thicker and blocked more light near the lighthouses.

The Analogy: Imagine you are standing near a busy, loud party (the quasar). You might expect the noise to clear the air, but instead, you find that the air is actually more crowded with people.
The Reason: The lighthouses (quasars) don't just sit in empty space; they live in the densest, most crowded neighborhoods of the universe. The gas is naturally clumped together in these areas. The researchers found that this crowding of gas was so strong that it overpowered the "clearing" effect of the lighthouse's radiation. The gas was just too dense to be cleared out easily.

3. The Mystery: Brightness Doesn't Matter

The researchers then asked a logical question: "If the lighthouse is brighter, shouldn't it clear the fog more?"

They split the quasars into groups: dim ones, medium ones, and super-bright ones. They expected the super-bright ones to have the clearest bubbles of fog.
The Result: They found no difference. The fog looked exactly the same around the dim lighthouses as it did around the super-bright ones.

Why is this strange?
If you have a stronger flashlight, you expect to see further. The fact that the brightness didn't change the fog suggests a few possible explanations:

The "Double-Edged Sword": Maybe the brightest lighthouses live in the densest neighborhoods. The extra gas density cancels out the extra brightness, leaving the fog looking the same.
The "Flickering Light": Maybe the lighthouses aren't steady. They might flash on and off very quickly. If a lighthouse was super bright yesterday but is dim today, the fog might still be "remembering" the old brightness, or the light we see now isn't the same light that cleared the fog.
The "Directional Beam": Maybe the lighthouses don't shine in all directions. They might be like a laser pointer, shooting a beam in one direction while the sides remain dark. If we are looking at the "side" of the lighthouse, its brightness doesn't matter because the beam isn't hitting the fog there.

4. What This Tells Us

This study is a bit like trying to figure out how a lighthouse works by looking at the water around it, but the water is so choppy and crowded that it's hard to tell if the light is doing anything.

The Environment Wins: The most important takeaway is that the environment (the crowded gas) is the main player, not just the radiation from the black hole.
The "Luminosity" Puzzle: The fact that brightness doesn't change the fog suggests that our simple idea of "brighter = more clearing" is too simple. It hints that quasars might be changing their brightness rapidly, or beaming their light in specific directions, or that they live in such dense places that the gas density masks their power.

Summary

In short, the researchers used a giant telescope to look at 10,000 pairs of cosmic lighthouses. They expected the bright ones to clear the cosmic fog around them. Instead, they found the fog was always thick because the lighthouses live in crowded neighborhoods. Furthermore, the brightness of the lighthouse didn't seem to matter at all, suggesting that these cosmic beacons might be flickering, beaming their light in secret directions, or living in such dense gas that their power is hidden from us.

Technical Summary: Measurements of Quasar Proximity Zones with the Ly $\alpha$ Forest of DESI Y1 Quasars

Problem Statement
The intergalactic medium (IGM) surrounding quasars is shaped by a complex interplay between the quasar's ionizing radiation and the dense galactic environments in which they reside. This region, known as the "quasar proximity zone," offers a unique probe of quasar physics, including their lifetimes, emission anisotropy, and the mass of their host halos. However, previous studies of the transverse proximity effect (TPE)—Ly $\alpha$ absorption in background sightlines near foreground quasars—have yielded inconclusive results. Some studies reported enhanced absorption (suggesting dense environments or short lifetimes), while others found reduced absorption (suggesting ionization). A major limitation of prior work has been small sample sizes, preventing a detailed statistical investigation of how the proximity signal depends on quasar luminosity, redshift, and transverse separation.

Methodology
This study utilizes the Dark Energy Spectroscopic Instrument (DESI) Year 1 (Y1) data release, specifically the first public data release (DR1), to construct an unprecedented sample of over 10,000 quasar pairs. The analysis focuses on pairs with projected comoving transverse separations ( $r_\perp$ ) less than $2 \, h^{-1}\text{Mpc}$ .

Data Selection: The authors selected 428,403 quasars from the DESI Ly $\alpha$ forest catalog (Region A: rest-frame 1040–1205 Å). They constructed a sample of 10,263 valid quasar pairs where the foreground quasar's proximity region ( $\pm 50 \, h^{-1}\text{Mpc}$ along the line of sight) is fully covered by the background quasar's spectrum.
Subsampling: To isolate physical dependencies, the sample was divided into three categories:
- Transverse Distance: Binned into three equal-width ranges ( $0.00\text{--}0.67$ , $0.67\text{--}1.33$ , $1.33\text{--}2.00 \, h^{-1}\text{Mpc}$ ).
- Redshift: Binned into three groups covering $z \approx 2.1\text{--}3.6$ .
- Luminosity: Binned by monochromatic luminosity at 1500 Å ( $\log(\nu L_\nu)$ ), with bins carefully constructed to maintain similar redshift distributions to avoid degeneracy.
Control Sample: A control sample of background quasars was generated by matching the redshift and signal-to-noise ratio (S/N) of the proximity sample to isolate the specific effect of the foreground quasar from general IGM evolution.
Analysis: The study measured the stacked Ly $\alpha$ transmitted flux ( $f_q/C_q$ ) and derived the optical depth ( $\tau = -\ln(f_q/C_q)$ ). The observed optical depth ratio ( $\tau_{prx}/\tau_{IGM}$ ) was compared against theoretical predictions based on photoionization equilibrium. The model accounts for the uniform UV background ( $\Gamma_{UVB}$ ), the specific ionizing flux from the foreground quasar ( $\Gamma_q$ ), and the gas overdensity ( $\Delta$ ).

Key Results

Dominance of Gas Clustering: The primary finding is that Ly $\alpha$ absorption is enhanced in the vicinity of foreground quasars compared to the control sample. This indicates that the clustering of neutral hydrogen (H I) in the dense environments surrounding quasars dominates over the ionizing effect of the quasar's radiation, which would otherwise reduce absorption.
Overdensity Profiles: By inverting the optical depth ratio, the authors inferred gas overdensity profiles. They found overdensities as high as $\Delta \sim 10$ at comoving distances of $\sim 1 \, h^{-1}\text{Mpc}$ from the most luminous systems. The overdensity profiles are consistent across different luminosity bins when the contribution of quasar ionization is removed, suggesting the underlying density structure is robust.
Lack of Luminosity Dependence: Contrary to the expectation that more luminous quasars should produce a larger ionized region (lower optical depth), the study finds no significant dependence of the proximity profile on the foreground quasar's luminosity. The observed optical depth remains similar across bins spanning nearly an order of magnitude in luminosity.
Transverse Distance Dependence: The enhanced absorption is strongest for sightlines closest to the foreground quasars ( $r_\perp < 0.67 \, h^{-1}\text{Mpc}$ ), consistent with the expectation that the ionizing flux drops below the UV background at larger separations.
Symmetry: The analysis of near-side ( $r_\parallel < 0$ ) versus far-side ( $r_\parallel > 0$ ) transmission reveals no statistically significant asymmetry. While a slight visual asymmetry exists, it is not robust enough to definitively constrain quasar lifetimes or emission geometry in the stacked sample.

Interpretation and Significance
The paper posits that the lack of luminosity dependence in the observed optical depth likely arises from one of two scenarios:

Cancellation: A coincidental balance where the higher ionizing flux of luminous quasars is exactly offset by their residence in significantly denser environments (higher gas overdensity). However, the authors note this conflicts with other clustering studies suggesting weak luminosity dependence in quasar environments.
Emission Properties: The observed luminosity may not trace the ionizing flux reaching the transverse sightlines. This could be due to:
- Time Variability: Quasars may undergo rapid luminosity changes (episodic lifetimes) shorter than the light-crossing time of the transverse separation, decoupling the observed flux from the historical flux that ionized the transverse gas.
- Anisotropy: Quasar radiation may be emitted in bipolar cones. If the transverse sightlines lie outside these cones, the ionizing flux they receive would be minimal regardless of the quasar's observed luminosity.

Significance
This work represents the first high-precision measurement of the TPE using a sample size nearly 50 times larger than previous SDSS-based analyses. By leveraging the statistical power of DESI, the study confirms that dense environments are the primary driver of Ly $\alpha$ absorption in quasar proximity zones at $z \sim 2\text{--}3.5$ . The results highlight the necessity of modeling both gas overdensity and quasar emission geometry simultaneously to interpret proximity effects. The authors conclude that future analyses, potentially combining TPE with line-of-sight proximity effect (LPE) measurements and utilizing upcoming DESI data releases, are required to disentangle the effects of quasar lifetimes, anisotropy, and environmental density.

1. The Setup: A Game of "Look Behind the Lighthouse"

2. The Big Surprise: The Fog is Thicker, Not Clearer

3. The Mystery: Brightness Doesn't Matter

4. What This Tells Us

Summary

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