Search for quasar pairs with Gaia astrometric data III. Confirmation of 16 dual quasars and 36 projected quasars

This paper reports the spectroscopic confirmation of 16 dual quasars and 36 projected quasars using DESI DR1 data, highlighting a high-confidence strong gravitational lensing system and several close pairs that offer unique opportunities to study galaxy mergers and the circumgalactic medium.

The Gist

Imagine the universe as a giant, cosmic dance floor. In the center of this dance floor, massive galaxies are constantly bumping into each other, merging, and swirling together. At the heart of almost every galaxy sits a supermassive black hole, a gravitational monster that eats anything that gets too close.

When two galaxies crash into each other, their central black holes usually start dancing a duet. Eventually, they might get close enough to form a "binary" pair, spiraling toward each other until they finally smash together. This collision is one of the most violent events in the universe, creating ripples in space-time called gravitational waves.

The Problem:
Finding these "dancing pairs" is incredibly hard. It's like trying to spot two fireflies buzzing right next to each other in a dark forest, but from miles away. Often, what looks like two fireflies is actually just one firefly seen through a funhouse mirror (a gravitational lens) or two completely unrelated fireflies that just happen to be in the same line of sight (a projection).

The Solution:
The authors of this paper are a team of astronomers from Beijing Normal University. They decided to play detective using a new set of tools. They took a massive list of potential "quasar pairs" (quasars are the bright, active hearts of galaxies) and used data from the Gaia satellite (which maps the positions of stars and galaxies with extreme precision) to filter out the fakes.

They used a clever trick: Real quasars are so far away that they don't move across the sky. If a candidate object does move, it's likely a nearby star, not a distant quasar. By filtering for objects that stand perfectly still, they narrowed down their search to the most promising suspects.

The Hunt:
They then took these suspects to the DESI telescope, a giant machine that can take "fingerprints" (spectra) of millions of objects at once. By analyzing these fingerprints, they could tell exactly how fast the objects were moving away from us and how far away they were.

The Findings:
After all the filtering and checking, they confirmed 52 new pairs. Here is the breakdown:

1. 16 True "Dancing Pairs" (Dual Quasars):
   These are the real deal. Two black holes, each in its own galaxy, that are actually close neighbors in space (within about 100,000 light-years of each other). They are likely in the early stages of a galactic merger.

   • Analogy: Imagine finding two couples actually holding hands and dancing together in the crowd, rather than just two people standing near each other by chance.

2. 36 "Look-Alikes" (Projected Quasars):
   These are imposters. They look like a pair from our perspective, but one is actually much farther away than the other. They are just aligned by chance, like two cars on a highway that look close together because one is far ahead and the other is far behind, but they are in different lanes.

   • Why they matter: Even though they aren't a pair, the closer one acts like a flashlight shining through the gas cloud of the farther one. This helps astronomers study the invisible gas surrounding galaxies.

The Star of the Show: J0023+0417
One specific system they found, named J0023+0417, is a special case. The two quasars look almost identical in their light signatures, and there appears to be a galaxy sitting right between them.

• The Metaphor: This is likely a cosmic mirage. A massive galaxy in the foreground is acting like a giant lens, bending the light of a single background quasar and splitting it into two images. It's like looking at a single streetlamp through a thick glass prism and seeing two lamps instead of one. The team is now working to confirm if this is indeed a "gravitational lens" or a true twin.

Why This Matters:
This paper is like adding 16 new pieces to a giant puzzle. Every time we find a confirmed dual quasar, we learn more about how galaxies grow and how black holes merge. These mergers are the "birth cries" of the gravitational waves that future space detectors (like LISA) hope to hear.

In short, the team used a "stillness filter" and a "fingerprint scanner" to separate the real cosmic dancers from the look-alikes, giving us a clearer view of the universe's most dramatic collisions.

Technical Summary

1. Problem Statement

Dual quasars (pairs of active galactic nuclei) separated by kiloparsec scales are considered precursors to binary supermassive black holes (SMBHs) and serve as critical probes for understanding galaxy merger dynamics and SMBH coalescence. However, the census of confirmed dual quasars remains small (~200 systems) due to several challenges:

• Angular Resolution: Small separations challenge ground-based imaging and spectroscopy.
• Contamination: Candidate selection is plagued by two major contaminants:
  1. Gravitationally Lensed Quasars: Multiple images of a single background quasar mimicking a dual system.
  2. Projected Quasar Pairs: Unrelated quasars at different redshifts appearing close on the sky by chance alignment.
• Selection Biases: Traditional methods (e.g., double-peaked emission lines or color selection) often suffer from confusion with complex single AGN kinematics or stellar contamination.

2. Methodology

The authors employed a multi-stage selection and confirmation strategy building upon their previous work (the Milliquas Gaia Quasar Pair Candidates, or MGQPC, catalog):

• Candidate Selection (Astrometry): The study utilizes the MGQPC catalog (~4,000 candidates), which selects pairs based on zero proper motion and zero parallax derived from Gaia astrometric data. This method effectively isolates distant quasars from foreground stars, which typically exhibit measurable proper motion.
• Spectroscopic Data Sources:
  • Primary Source: Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1), providing high-efficiency spectra for ~1.6 million quasars.
  • Secondary Sources: SDSS DR16 (for cross-validation and redshift consistency) and the Strong Gravitational Lensing Database (SLED) to exclude known lenses.
• Classification Criteria:
  • Dual Quasars: Defined by a radial velocity difference $|\Delta v_r| \le 2000 \text{ km s}^{-1}$ and consistent redshifts.
  • Projected Quasars: Defined by $|\Delta v_r| > 2000 \text{ km s}^{-1}$, indicating distinct redshifts.
  • Visual Inspection: Crucial for correcting automated redshift errors, identifying Broad Absorption Line (BAL) features, and distinguishing between dual systems and gravitational lenses (by checking for deflector galaxies in DESI Legacy Imaging Surveys images).
• Filtering Process: The team cross-matched 236 DESI-confirmed members against existing literature (Jing et al. 2025a) and SLED, removing duplicates and known lenses to isolate 52 new systems.

3. Key Contributions

• Spectroscopic Confirmation: This is the third paper in the series, providing the first large-scale spectroscopic confirmation of MGQPC candidates using DESI DR1 data.
• Refined Classification: The study successfully distinguishes between true dual quasars, projected pairs, and potential lensing systems through a combination of velocity thresholds and visual spectral/imaging analysis.
• Catalog Update: The authors released MGQPC Version 2, a cleaned catalog of 3,643 candidates after removing confirmed systems, duplicates, and known contaminants.
• Discovery of a Strong Lens Candidate: Identification of a high-confidence strong gravitational lensing system (J0023+0417) serendipitously found during the dual quasar search.

4. Results

The study confirmed 52 new quasar pair systems:

A. Dual Quasars (16 Systems)

• Redshift Range: $z = 0.609$ to $2.758$(Median$z \approx 1.46$).
• Separation: Angular separations range from $\sim 4''$ to $12''$, corresponding to projected physical distances ($r_p$) of roughly$37$to$97$ kpc.
• Notable Systems:
  • J0023+0417: Exhibits nearly identical spectral features (including C IV absorption and Mg II self-absorption) and a very small redshift difference ($\Delta z = 0.0031$). A red object is visible between the members. While initially a candidate, the authors classify it as a high-confidence strong gravitational lens due to the spectral homology and the presence of a potential deflector, despite the large separation ($8.98''$).
  • J1613+1611 & J0126+1955: Identified as distinct dual quasars where one member shows BAL features while the other does not, ruling out a simple lensing scenario.
• Significance: The sample probes the "cosmic noon" ($z \sim 1-3$), a peak epoch for star formation and SMBH accretion, offering insights into SMBH pairing during major galaxy mergers.

B. Projected Quasars (36 Systems)

• Redshift Range: $z = 0.57$ to $3.07$.
• Separation: Projected distances ($r_p$) range from $20.46$to$99.17$ kpc.
• Scientific Utility:
  • CGM Probes: Four systems have projected distances $< 30$ kpc. These provide unique "background sightlines" to study the circumgalactic medium (CGM) and gas outflows of the foreground host galaxy via absorption lines.
  • Lyman-alpha Forest: Systems with high-redshift backgrounds (e.g., J0909+1509) offer opportunities to test the transverse proximity effect and gas distribution in the intergalactic medium (IGM) near foreground quasars.

5. Significance

• Population Statistics: The confirmation of 16 new dual quasars significantly expands the known population, aiding statistical studies of SMBH merger rates and the efficiency of gas-fueled accretion during galaxy interactions.
• Methodological Validation: The study validates the "zero proper motion" astrometric selection strategy as a robust alternative to color-based methods, effectively reducing stellar contamination.
• Gravitational Lensing: The discovery of J0023+0417 highlights the potential of dual quasar searches to serendipitously find wide-separation strong lenses, which are valuable for precision cosmology and mass reconstruction.
• Future Prospects: The identified close projected pairs serve as prime targets for future high-resolution spectroscopy to characterize the multiphase gas in quasar host halos, while the dual quasar sample provides targets for future space-based gravitational wave detectors (e.g., LISA) and pulsar timing arrays.