Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. I. Method and Candidates

This paper presents a systematic method using ${\it Gaia}$ astrometric data to identify 4,062 new quasar pair candidates within 100 kpc of known quasars, significantly expanding the available sample for studying quasar interactions and galaxy evolution.

The Gist

Imagine the universe as a giant, cosmic dance floor. Most of the time, the dancers (galaxies and their supermassive black holes) are far apart, doing their own thing. But sometimes, two dancers bump into each other, grab hands, and start spinning together. In astronomy, when two super-bright "stars" (called Quasars) are found dancing this close together, we call them a Quasar Pair.

Finding these pairs is like trying to spot two specific fireflies in a massive, dark forest. They are incredibly rare, and until now, we've only found about 160 confirmed pairs. This paper is about a team of astronomers who built a new, super-powered net to catch thousands more of these elusive pairs.

Here is the story of how they did it, broken down into simple steps:

1. The Problem: The "Cosmic Needle in a Haystack"

Quasars are the bright hearts of distant galaxies. Usually, they are alone. But when galaxies crash into each other, their black holes can wake up and shine as quasars, creating a pair.

• Why do we care? These pairs are like time machines. They tell us how galaxies merge, how black holes grow, and they might even be the source of "gravitational waves" (ripples in space-time) that we are trying to detect.
• The difficulty: The universe is huge. Most methods to find them rely on guessing their colors or looking for weird light patterns, which often leads to false alarms (like mistaking a regular star for a quasar).

2. The Solution: The "Motionless Ghost" Trick

The team used data from Gaia, a European space telescope that acts like a 3D map of the sky.

• The Analogy: Imagine you are looking at a busy street. Cars (stars in our galaxy) are zooming past you; they have speed and direction. But a distant mountain (a quasar) is so far away that it looks like it isn't moving at all, even though it's actually moving incredibly fast.
• The Method: The astronomers looked for "ghosts" in the sky. They searched for objects that:
  1. Are near a known quasar (within a distance of 100,000 light-years).
  2. Have zero speed across the sky (no "proper motion").
  3. Have zero parallax (no shift in position when viewed from different angles, meaning they are incredibly far away).

If a star is moving, it's a local star. If it's perfectly still, it's likely a distant quasar!

3. The Process: From Millions to Thousands

The team followed a three-step recipe:

1. The Big Match: They took a list of 1 million known quasars (the "Million Quasar Catalog") and checked the Gaia map for any "still" objects nearby.
2. The Filter: They used math to filter out anything that wasn't perfectly still. This narrowed the list down to about 5,800 candidates.
3. The Human Eye (Visual Inspection): This is the most crucial step. Computers are great, but they get confused by crowded star fields (like looking at a dense forest where it's hard to tell one tree from another). The team looked at pictures of these 5,800 candidates and manually removed the ones that were just messy star clusters or nearby galaxies.

4. The Result: A Treasure Trove

After all the filtering and human checking, they found 4,062 new quasar pair candidates.

• The New Record: Before this, we knew of about 160 pairs. This catalog adds thousands of new possibilities.
• The Comparison: They compared their list with three other major lists of quasar pairs. They found that their new list was mostly different from the others. It's like they found a whole new neighborhood of houses that the other maps missed.
• The "Wide" Pairs: Interestingly, their method found pairs that are quite far apart (several arcseconds). Other methods usually only find pairs that are very close together. This is like finding couples who are holding hands across the room, not just those hugging tightly.

5. What's Next?

The team has a list of 4,000+ suspects, but they aren't 100% sure yet. They need to take "spectroscopic" pictures (like taking a fingerprint of the light) to confirm that these are indeed two quasars and not just two random objects that happen to look alike.

They are already using giant telescopes in China and the US to check these candidates. If successful, this work will revolutionize our understanding of how galaxies and black holes evolve, giving us a much clearer picture of the universe's "family tree."

In a nutshell: The astronomers used the fact that distant objects don't seem to move to find thousands of new "cosmic couples" (quasar pairs) that were hiding in plain sight, opening a new chapter in understanding how the universe builds itself.

Technical Summary

Here is a detailed technical summary of the paper "Search for Quasar Pairs with Gaia Astrometric Data. I. Method and Candidates" by Chen et al. (2025).

1. Problem Statement

Quasar pairs (systems of two interacting quasars) are critical laboratories for studying galaxy mergers, supermassive black hole (SMBH) co-evolution, and the origins of nanohertz gravitational waves. However, confirmed quasar pairs at kiloparsec (kpc) scales are extremely rare in the universe. Existing catalogs are limited in size and often biased toward specific selection methods (e.g., color-based or lensing-based searches). The scarcity of samples hinders statistical exploration of quasar interactions and the "final parsec problem" (the mechanism by which SMBH binaries merge). There is a pressing need for a systematic, unbiased method to discover new quasar pair candidates, particularly those with larger separations (up to 100 kpc) that may have been missed by previous surveys.

2. Methodology

The authors developed a systematic astrometric search pipeline combining the Million Quasar Catalog (MQCv8) and Gaia Data Release 3 (DR3). The methodology proceeds in three main stages:

A. Data Preparation and Cross-Matching

• Base Sample: Selected 908,990 quasars from MQCv8 with redshift $z \ge 0.5$.
• Gaia Matching: Cross-matched these quasars with Gaia DR3 within a $1''$ radius to obtain optical counterparts, resulting in 582,725 quasars with Gaia astrometry.
• Search Radius Calculation: Calculated the angular separation corresponding to a physical transverse distance of 100 kpc for each quasar based on its redshift (assuming a flat $\Lambda$CDM cosmology).
• Candidate Extraction: Searched for Gaia sources within this 100 kpc radius around each known quasar. After excluding sources with existing spectroscopic redshifts (to avoid double-counting known pairs), an input catalog of 85,913 sources (named astrometricCAT) was isolated.

B. Astrometric Filtering

To distinguish extragalactic sources (quasars) from Galactic contaminants (stars), the authors applied probability-based astrometric criteria derived from reference samples of SDSS DR18 quasars (GoodQSO) and stars (GoodSTAR).

• Criteria: Utilized the probability density of zero proper motion ($f_{0pm}$) and zero parallax ($f_{0plx}$).
• Thresholds: Sources were selected if:
  1. $\log f_{0pm} \ge -2$
  2. $\log f_{0plx} \ge -2$
• Performance: These criteria achieved a purity of $\sim 89\%$ and completeness of $\sim 97\%$ in isolating quasars from stars. This step yielded 5,851 preliminary extragalactic candidates (MGQPCpreCAT).

C. Visual Inspection

• Morphological Classification: The astrometric filter cannot distinguish between point-source quasars and extended galaxies.
• Imaging Data: Used pseudo-color images from DESI Legacy Imaging Surveys (DR10) and Pan-STARRS (for regions with poor DESI coverage or near the Galactic plane).
• Process: Manual inspection of the 5,851 candidates to remove:
  • Dense stellar fields (Galactic plane, clusters).
  • Nearby galaxies (extended sources).
• Scope: Focused on the region $\text{Dec} \ge -30^\circ$ (Pan-STARRS footprint) for the first stage of inspection.
• Result: Removed 106 contaminated cases, leaving 4,062 final quasar pair candidates.

3. Key Contributions

1. Systematic Astrometric Search: Demonstrated a robust, unbiased method for finding quasar pairs by leveraging Gaia's high-precision astrometry (zero proper motion/parallax) rather than relying solely on color or photometric variability.
2. New Catalog (MGQPC): Created the MGQPC catalog containing 4,062 quasar pair candidates.
3. Complementarity: Showed that the MGQPC catalog significantly complements existing catalogs (Dawes23, He23, Wu24).
   • Overlap: Only 98 candidates overlapped with the three major existing catalogs.
   • Novelty: Identified 3,964 previously uncataloged candidates.
4. Wider Separation Coverage: Unlike many existing catalogs that focus on sub-arcsecond or lensed pairs, MGQPC effectively targets separations up to $\sim 18''$ (median $\sim 8.8''$), covering the 100 kpc physical scale crucial for studying galaxy-scale interactions.

4. Key Results and Statistics

• Sample Size: 4,062 candidates (MGQPCs).
• Median Properties:
  • Member Separation: $8.81''$(significantly larger than the$\sim 2''$ median of comparison catalogs).
  • Gaia G-band Magnitude: $20.49$ (slightly fainter/deeper than other catalogs).
  • Redshift: $1.59$(peaking around the "Cosmic Noon" epoch,$z \sim 0.5 - 2.5$).
• Distribution: The catalog is dominated by faint sources and moderately large separations, filling a gap in the parameter space of quasar pair searches.
• Interesting Candidates:
  • Identified several Wide-Separation Lensed Quasar (WSLQ) candidates with separations of $6''-10''$ and potential foreground lenses.
  • Found potential "quasar-galaxy-quasar" projection systems, which are valuable for studying the circumgalactic medium.

5. Significance

• Statistical Power: The discovery of nearly 4,000 new candidates provides a massive dataset for statistical studies of quasar clustering, merger rates, and environmental overdensities.
• Gravitational Waves: By expanding the sample of potential SMBH binaries (especially at kpc scales), this work aids in understanding the population of sources for future nanohertz gravitational wave detectors (e.g., PTA).
• Methodological Advancement: The paper validates that combining large quasar catalogs with Gaia's astrometric precision is a highly efficient way to filter out stellar contamination and identify extragalactic pairs, offering a template for future surveys.
• Future Work: The authors note that extensive spectroscopic follow-up (using telescopes like Xinglong 2.16m, LAMOST, and Palomar 200-inch) is underway to confirm the redshifts and physical nature of these candidates, which will be detailed in subsequent papers.

In summary, this work represents a major step forward in populating the census of quasar pairs, moving beyond the limitations of previous color-based or lensing-focused searches to provide a comprehensive, astrometrically vetted sample for the next generation of astrophysical research.