Confirming lensed-quasar candidates with DESI and P200 spectroscopy I. 14 lensed quasars and 8 lensed galaxies

This paper reports the confirmation of two new lensed quasars and the identification of 12 likely lensed quasars and eight new lensed galaxies by cross-matching 1,724 candidates from imaging surveys with DESI DR1 and P200 spectroscopy, demonstrating the efficiency of wide-field spectroscopic surveys in validating strong lens systems.

The Gist

Imagine the universe as a giant, cosmic funhouse mirror. Sometimes, a massive object like a galaxy sits between us and a distant, brilliant light source (like a quasar, which is a super-bright black hole). This massive object bends the light, acting like a lens, and creates multiple images of that single light source. It's like looking at a candle through a wine glass and seeing four reflections instead of one.

These "lensed quasars" are incredibly valuable to astronomers. They act as natural telescopes, helping us map out invisible dark matter, measure the expansion of the universe, and study how galaxies evolve. But finding them is like finding a needle in a haystack, and proving they are real (and not just two different stars that happen to look close together) is even harder.

This paper is a report card from a team of astronomers who went on a hunt for these cosmic mirages. Here is what they did, explained simply:

1. The Great Filter (The Search)

The team started with a massive "Wanted" list of 1,724 potential lensed quasars. These candidates were found by previous surveys using powerful cameras (like KiDS, HSC, and DESI-LS) that take pictures of the sky. However, a picture isn't enough; you need to know what the object is.

To solve this, they used DESI, a massive spectroscopic survey. Think of a spectrograph as a prism that splits light into a rainbow. By looking at the rainbow, astronomers can tell exactly how fast an object is moving away from us (its redshift) and what it's made of.

• The Challenge: DESI has thousands of tiny fiber-optic "straws" that suck up light from the sky. Sometimes, two quasar images are so close together that one straw catches both, or the straw misses one entirely.
• The Result: They matched their "Wanted" list against the DESI data and found spectra (light fingerprints) for 677 unique systems.

2. The Detective Work (Confirmation)

To be sure a system is a real lens, you usually need to prove that the two (or more) images are actually the same object. This means they must have the exact same "fingerprint" (redshift).

• The Palomar 200-inch Telescope (P200): When DESI's "straws" weren't enough, the team used a giant telescope in California with a long slit (like a long, thin window) to look at specific targets. This allowed them to see the light from both images separately, even if they were very close together.
• The Verdict:
  • 2 Confirmed: They found two systems where they could prove, beyond a doubt, that the images were the same quasar. One had a separation of about 0.4 arcseconds (tiny!), and the other was about 1 arcsecond.
  • 12 "Likely" Candidates: They found 12 more systems that look exactly like lenses. They have the right shape, the right colors, and at least one confirmed spectrum. They just need one more piece of the puzzle (a second spectrum) to officially confirm them. It's like seeing a suspect's car and license plate, but you haven't caught the driver yet.

3. The Bonus Round (Static Lenses)

While hunting for the moving "quasar" lights, they also found 8 static strong lenses. These are galaxies or groups of galaxies that are lensing other galaxies (not quasars).

• Think of these as the "background scenery" that got distorted.
• They confirmed these by getting spectra for both the lensing galaxy (the one doing the bending) and the background galaxy (the one getting bent).

4. Why This Matters (The "So What?")

• Efficiency: This paper shows that wide-field spectroscopic surveys like DESI are incredibly efficient at finding these objects. It's like using a metal detector on a beach instead of digging holes by hand.
• The "Blended" Problem: A major takeaway is that many lenses are hard to confirm because the images are too close together. The team found that sometimes one telescope (DESI) sees the light from both images mixed together, while another (P200) can separate them. Using both tools together is the key to unlocking these secrets.
• Future Goldmines: The 12 "likely" candidates are a treasure trove for future telescopes. Once we get the missing spectra, we'll have a new batch of tools to measure the Hubble Constant (how fast the universe is expanding) and study dark matter.

The Bottom Line

This paper is a successful "mop-up" operation. The team took a huge list of suspects, used the best available tools (DESI and Palomar) to interrogate them, and successfully confirmed two new cosmic mirages while identifying 12 more that are almost certainly real. They also found 8 new galaxy lenses along the way.

It's a reminder that in astronomy, the most exciting discoveries often happen when you combine different tools to look at the same patch of sky, turning a blurry guess into a confirmed fact.

Technical Summary

Here is a detailed technical summary of the paper "Confirming lensed-quasar candidates with DESI and P200 spectroscopy: I. 14 lensed quasars and 8 lensed galaxies" by He et al. (2026).

1. Problem Statement

Strongly lensed quasars are critical astrophysical tools for studying cosmology (e.g., measuring the Hubble constant), the co-evolution of supermassive black holes and host galaxies, and dark matter distribution. However, the number of confirmed lensed quasars remains low compared to the thousands of candidates identified by wide-field imaging surveys (such as KiDS, HSC, and DESI-LS). The primary bottleneck is spectroscopic confirmation, which requires obtaining spectra for multiple images of the same quasar to verify they share the same redshift and distinguishing them from false positives (e.g., projected pairs of unrelated quasars or stars). Existing surveys often lack the spatial resolution or fiber allocation to capture multiple images of close-separation systems simultaneously.

2. Methodology

The authors employed a multi-stage approach combining large-scale imaging catalogs, high-efficiency spectroscopy, and targeted follow-up observations:

• Candidate Compilation: They aggregated previously identified lensed-quasar candidates from four major surveys: KiDS, HSC-SSP, and DESI Legacy Imaging Surveys (DESI-LS). After de-duplicating entries within a 6" radius, they created a consolidated catalog of 1,724 unique systems.
• DESI DR1 Cross-Matching: They cross-matched these candidates with the Dark Energy Spectroscopic Instrument (DESI) Data Release 1 (DR1).
  • Result: 973 DESI spectra were retrieved for 677 unique systems (457 with single spectra, 220 with multiple spectra).
• Targeted Follow-up (P200/DBSP): To address systems where DESI fibers could not resolve multiple images or where spectra were missing, the team conducted optical spectroscopy on September 4, 2024, using the Palomar 200-inch (P200) Hale Telescope with the Double Spectrograph (DBSP).
  • They observed 10 candidates, using a long slit (1.5") to simultaneously capture spectra of both quasar images in close pairs.
• Data Analysis & Modeling:
  • Spectroscopy: Redshifts ($z_s$ for source, $z_d$ for lens) were determined using emission/absorption lines (e.g., Ly$\alpha$, CIV, MgII) from both DESI and DBSP.
  • Light Modeling: They performed 2D light-profile fitting using lenstronomy, modeling the system as a Sérsic profile for the lens galaxy plus Point Spread Function (PSF) components for the quasar images. This allowed for the deblending of components and photometric redshift ($z_{phot}$) estimation for lens galaxies using EAZY.
  • Mass Modeling: They applied a Singular Isothermal Ellipsoid (SIE) model to the image configurations to derive Einstein radii ($\theta_E$) and ellipticity parameters, verifying if the geometry is consistent with gravitational lensing.

3. Key Contributions

• Confirmation of New Systems: The paper confirms 2 new lensed quasars and identifies 12 "likely" lensed quasars that meet most criteria but require further spectroscopy for final confirmation.
• Static Strong Lenses: The authors report 8 new static strong lenses (galaxy- to group-scale) discovered incidentally during the quasar search, expanding the sample of confirmed lensed galaxies.
• Methodological Demonstration: The study highlights the complementary nature of wide-field spectroscopy (DESI) and targeted long-slit spectroscopy (P200/DBSP). It demonstrates that DESI can confirm small-separation lenses if the fiber accidentally captures multiple images, while long-slit spectroscopy is essential for resolving blended systems.
• Public Data Release: The authors released the consolidated candidate catalog, the cross-matched DESI table, and detailed modeling parameters for all reported systems.

4. Key Results

A. Confirmed Lensed Quasars (2 Systems)

1. DESI-LS J0642+5617:
   • Source Redshift ($z_s$): 1.9264
   • Lens Redshift ($z_d$): $0.78^{+0.18}_{-0.04}$ (photometric)
   • Einstein Radius: $0''.39$ (smallest in the sample)
   • Confirmation: Confirmed via a single DESI fiber spectrum that captured both images, showing a blended QSO signature with no redshift difference.
2. DESI-LS J1800+5305:
   • Source Redshift ($z_s$): 3.2304
   • Lens Redshift ($z_d$): $0.8276^{+0.0004}_{-0.0164}$
   • Einstein Radius: $1''.07$
   • Confirmation: Confirmed via combined P200/DBSP and DESI spectra. Both images (A and B) showed consistent redshifts ($\sim 3.23$) and a visible lens galaxy.

B. Likely Lensed Quasars (12 Systems)

These systems exhibit double-image configurations, visible lens galaxies, and SIE models that fit the geometry well. However, they lack secure confirmation because:

• Missing Spectra: 9 systems have spectra for only one image.
• Blended Spectra: 2 systems (J1809+5451 and J2225+0409) have atypical blended spectra where individual components could not be isolated due to small separation or bad pixels in DBSP data.
• Ambiguous Lens: 1 system (J0422-0447) has a putative lens galaxy that requires deeper imaging to confirm.

C. Static Strong Lenses (8 Systems)

The authors confirmed 8 galaxy- and group-scale lenses (e.g., HSC J0224-0336, J0913+0039).

• Redshifts: Lens redshifts ($z_d$) range from 0.41 to 0.61; source redshifts ($z_s$) range from 0.61 to 1.67.
• Caveats: Some source redshifts are tentative due to low signal-to-noise ratios or contamination by sky lines (e.g., J0921+0444, J2305-0002).

5. Significance and Implications

• Efficiency of Spectroscopic Surveys: The study underscores the efficiency of DESI in confirming lensed systems but also highlights a selection bias: DESI is more efficient at confirming large-separation systems or those where fibers happen to cover multiple images. Small-separation lenses often require targeted long-slit spectroscopy (like P200/DBSP) for confirmation.
• Future Cosmology Targets: The newly confirmed systems, particularly those with high lens redshifts (e.g., $z_d \approx 0.98$ for J2348+1530), provide valuable targets for studying galaxy formation at earlier cosmic times.
• Time-Delay Cosmology: Many of these systems fall within the footprint of the Wide Field Survey Telescope (WFST). Systems with image separations $>1''.2$ are suitable for time-delay measurements, which are crucial for independent measurements of the Hubble constant ($H_0$).
• Synergy with Future Facilities: The sample is poised for follow-up with space-based imaging (Euclid, CSST, Roman) to resolve lens arcs and improve mass modeling, as well as future spectroscopic surveys (4MOST, MUST) to resolve the remaining "likely" candidates.

In summary, this paper significantly expands the census of confirmed lensed quasars and static lenses, demonstrating a robust workflow for moving from imaging candidates to spectroscopically confirmed systems using a combination of large-scale survey data and targeted follow-up.