A Quasar Pair Sample Compiled from DESI DR1

This paper presents a statistically significant sample of 1,220 quasar pairs and candidates compiled from DESI DR1, which includes 1,020 confirmed pairs and a notable wide-separation quadruply lensed quasar candidate, providing a crucial dataset for studying galaxy mergers and black hole growth.

The Gist

Imagine the universe as a giant, bustling cosmic city. In this city, the most powerful "buildings" are quasars—super-bright lighthouses powered by supermassive black holes eating gas and dust. Usually, these lighthouses stand alone. But sometimes, two galaxies crash into each other, and their black holes get dragged along, creating a rare cosmic event: a Quasar Pair.

This paper is like a massive new census of these cosmic twins, conducted by a team of astronomers using a super-powered telescope called DESI (Dark Energy Spectroscopic Instrument).

Here is the story of their discovery, broken down into simple concepts:

1. The Great Cosmic Search

Before this study, astronomers had only found about 200 confirmed quasar pairs. It was like trying to study the mating habits of a rare bird by only spotting a few individuals in a forest. The sample was too small to draw big conclusions.

The researchers used the DESI DR1 catalog, which is like a massive phone book containing 1.6 million confirmed quasars. They wrote a computer program to act like a "matchmaker," scanning the list to find any two quasars that were:

• Close neighbors: Within a specific distance (about 110,000 light-years apart).
• Moving together: Not just passing by in the same direction, but actually traveling at similar speeds (like two cars driving side-by-side on a highway, rather than one speeding past the other).

2. The "Eye Test" (Visual Classification)

The computer found over 1,800 potential pairs, but computers can be fooled. Sometimes two stars look close together but are actually at vastly different distances (like a streetlamp and a distant airplane looking aligned).

So, the team did what astronomers call a "visual inspection." They looked at high-resolution photos and detailed light-spectra (the "fingerprints" of the light) for every candidate. They sorted them into three buckets:

• The Real Deal (QP - 1,020 pairs): These are genuine twins. They are close, moving together, and their "fingerprints" (spectra) are different, proving they are two distinct objects.
• The Maybe-So (QPC - 142 candidates): These look like twins, but they are so close together that the telescope's "fiber optic cable" (which collects the light) might have accidentally mixed their signals. They are likely real, but need a closer look to be 100% sure.
• The Illusions (LQC - 58 candidates): These aren't twins at all! They are lensed quasars. Imagine looking at a single streetlamp through a curved glass bottle; the light bends and creates multiple images. These are single quasars whose light has been bent by a massive galaxy in the foreground, making them look like pairs or even groups.

3. The Big Discoveries

Once they had their clean list of 1,220 systems, they found some fascinating patterns:

• The "Sweet Spot" Era: Most of these pairs exist when the universe was about 3 to 6 billion years old (redshift $z \sim 1–2.5$). This is the universe's "teenage years," a time when galaxies were crashing into each other like bumper cars, and black holes were feasting the most.
• The Frequency: They calculated that for every 10,000 quasars, about 0.6 are part of a pair. It's rare, but not that rare.
• The "Dance" of Mergers: About 64% of the pairs are moving very slowly relative to each other (less than 600 km/s). This suggests they are actually dancing together in a gravitational embrace, likely in the middle of a galaxy merger.
• The Wide-Separation Mystery: They found one incredibly strange system (J1011−0505). It looks like a single quasar that has been split into four images by gravity, but the images are spread out over a huge distance (7 arcseconds). Usually, these "Einstein Crosses" are tight little clusters. This one is so wide it suggests the "lens" bending the light isn't just a single galaxy, but perhaps a whole cluster of galaxies acting like a giant cosmic magnifying glass.

4. Why Does This Matter?

Think of galaxy mergers as the "birth of a new era" for black holes. When galaxies collide, gas gets dumped into the center, feeding the black holes and making them shine as quasars.

By studying these pairs, astronomers are essentially watching the "courtship" and "marriage" of galaxies in real-time. This helps them answer big questions:

• Do galaxy crashes trigger black hole growth? (Yes, it seems so).
• How do supermassive black holes eventually merge to create gravitational waves? (These pairs are the first step on that journey).

The Bottom Line

This paper is a massive upgrade to our "Quasar Pair" database. It moves us from a handful of case studies to a statistically robust sample of over 1,000 systems. It's like going from studying a single family to studying a whole city's census to understand how families form, grow, and interact.

The team has made their data public, inviting other scientists to use this new "map" to explore how the universe's most massive structures are built, one collision at a time.

Technical Summary

Here is a detailed technical summary of the paper "A Quasar Pair Sample Compiled from DESI DR1":

1. Problem Statement

Interacting quasar pairs (QPs), including dual and binary active galactic nuclei (AGNs), are critical tracers for understanding galaxy mergers, supermassive black hole (SMBH) growth, and large-scale structure formation. However, the current census of spectroscopically confirmed quasar pairs is small (fewer than 200 systems), limiting statistical significance. Existing samples suffer from:

• Heterogeneity: Constructed from serendipitous discoveries or specific high-resolution surveys, leading to selection biases.
• Contamination: High risk of false positives from stars blending with quasars or chance alignments along the line of sight.
• Lack of Uniformity: Previous studies often lack a systematic, homogeneous approach across a wide redshift range, particularly at high redshifts ($z \sim 1-2.5$) where quasar activity peaks.

2. Methodology

The authors constructed a large, homogeneous sample using the DESI DR1 (Data Release 1) quasar catalog, which contains 1.6 million spectroscopically confirmed quasars.

• Candidate Selection (Self-Matching):

  • The DESI DR1 quasar catalog was matched against itself.
  • Transverse Distance Criterion: A redshift-dependent angular threshold was applied to ensure a maximum physical transverse separation of $r_p \lesssim 110$ kpc.
  • Radial Velocity Criterion: A line-of-sight velocity difference threshold of $|\Delta V_r| \lesssim 2000$ km s$^{-1}$ was applied to distinguish physically associated pairs from random projections. This accounts for peculiar velocities and broad-line region redshift uncertainties.
  • This initial cut yielded 1,842 candidate systems.

• Visual Classification & Verification:

  • Candidates were visually inspected using DESI Legacy Imaging Surveys (LS-DR9) and SPARCL spectral data.
  • Classification Scheme:
    1. QP (Quasar Pair): Angular separation $\ge 2''$, distinct spectral features, no visible lensing galaxy. ($N=1020$)
    2. QPC (Quasar Pair Candidate): Angular separation $< 2''$ (near fiber diameter), distinct spectra but potential fiber spillover issues. ($N=142$)
    3. LQC (Lensed Quasar Candidate): Configurations showing lensing signatures (arcs, symmetry) and highly similar spectra. ($N=58$)
  • Contaminant Removal: Systems with very small separations ($\lesssim 1.2''$) showing identical spectra (likely duplicate observations) or poor spectral quality were excluded.

• Machine Learning (Appendix B):

  • The authors trained gradient-boosted classifiers (CatBoost and XGBoost) on the visually labeled sample to derive an empirical $r_p - |\Delta V_r|$ selection boundary. This "95% completeness envelope" offers a more robust selection function than simple flat cuts.

3. Key Contributions

• Largest High-Redshift Sample: The paper presents a sample of 1,220 quasar pairs/candidates, significantly expanding the known census.
• Homogeneity: The sample is derived from a single, uniform spectroscopic survey (DESI), reducing selection biases inherent in multi-survey compilations.
• Cross-Matching: The authors cross-matched their sample with existing databases (SLED, Big MAC, NED), identifying 145 previously known systems (23 confirmed lenses, 122 candidates), validating their classification scheme.
• Discovery of Rare Systems: Identification of a unique wide-separation ($\sim 7.15''$) quadruply lensed quasar candidate (J1011−0505), likely lensed by a galaxy group or cluster rather than a single galaxy.

4. Key Results

• Sample Statistics:

  • Total: 1,220 systems (1,020 QP, 142 QPC, 58 LQC).
  • Redshift Distribution: Peaks at $z \sim 1–2.5$, coinciding with the peak epoch of quasar activity ("cosmic noon").
  • Magnitude: Sources span $g, r, z \approx 17–24$ mag, peaking at 21–23 mag.
  • Angular Separation: Relatively uniform distribution between $2''$and$13''$, with an excess below$2''$ due to QPCs.

• Physical Association:

  • 63.8% of the QP sample has $|\Delta V_r| < 600$ km s$^{-1}$, strongly suggesting dynamical association rather than chance alignment.
  • The transverse distance distribution is roughly flat for $r_p \gtrsim 15$ kpc, suggesting sustained quasar fueling or gas replenishment throughout the merger cycle, rather than a sharp peak at small separations.

• Quasar Pair Fraction:

  • The overall pair fraction is estimated at $6.2^{+0.2}_{-0.2} \times 10^{-4}$.
  • The fraction shows weak redshift evolution, consistent with previous observational studies (e.g., Gaia, Subaru HSC) and theoretical simulations (Illustris/TNG), though slightly higher than some low-redshift studies, likely due to DESI's sensitivity to fainter targets.

• Notable Discovery (J1011−0505):

  • A candidate for a wide-separation quadruple lens. Unlike typical galaxy-scale lenses ($\sim 1-2''$), this system has a $\sim 7.15''$ separation, implying a massive deflector (group/cluster scale) at $z \sim 0.32$.

5. Significance

• Statistical Power: This sample provides the first statistically robust dataset for studying quasar pairs at high redshifts, enabling investigations into the merger sequence and SMBH co-evolution that were previously impossible due to small sample sizes.
• Merger Physics: The flat $r_p$ distribution and high fraction of low-velocity pairs support models where AGN activity is triggered and sustained over long merger timescales, not just during the initial close passage.
• Lensing Science: The inclusion of lensed quasar candidates, particularly the rare wide-separation quad, offers new targets for studying dark matter distributions and galaxy group/cluster potentials.
• Future Work: The sample serves as a foundation for future multi-wavelength follow-up (X-ray, radio) and high-resolution imaging to confirm physical associations and constrain AGN triggering mechanisms. The data is publicly available via GitHub.