Spectroscopic Redshifts for 461 Euclid Q1 Strong Gravitational Lenses from NISP Slitless Spectroscopy

Abstract: The Euclid Q1 Strong Lensing Discovery Engine identified 497 galaxy-scale lens candidates across 63 deg^2, yet none received spectroscopic characterization. We present the first spectroscopic redshifts for this sample from NISP SIR slitless spectroscopy (1.25-1.85 um, R~450) at zero additional telescope cost. Of 579 published Q1 lenses, 473 fall within SIR coverage. We detect emission lines in 461 systems and measure secure source redshifts (>=3 lines) for 419, deflector redshifts (>=2 absorption features) for 199, and complete (z_src, z_def) pairs for 178 systems (148 with dual-grism confirmation). Source redshifts span 0.70 < z_src < 2.88 (median 1.59); deflector redshifts span 0.24 < z_def < 2.47 (median 1.06). This is the largest single-campaign spectroscopic lens characterization to date, exceeding SLACS (85), BELLS (25), SL2S (~56), and AGEL (139) individually, with no dedicated follow-up time required. We present a quality-tiered catalog (148 gold-complete, 188 gold-source, 108 silver, 17 bronze) for mass modeling and lens statistics. Extrapolating to Euclid's 14,500 deg^2 wide survey implies ~100,000 spectroscopic lens redshifts -- three orders of magnitude beyond existing samples.

• The paper presents the first large-scale spectroscopic catalog of 461 Euclid Q1 lenses, including 178 systems with complete redshift pairs confirmed via dual-grism observations.
• The methodology employs optimized spectrum extraction, Gaussian fitting of multiple emission and absorption lines, and strict quality criteria to ensure robust redshift measurements.
• The catalog paves the way for improved mass modeling and large-scale structure analysis, with forecasts suggesting up to 100,000 lenses could be surveyed with future Euclid data.

Spectroscopic Redshifts for Euclid Q1 Strong Gravitational Lenses via NISP Slitless Spectroscopy

Overview

The paper "Spectroscopic Redshifts for 461 Euclid Q1 Strong Gravitational Lenses from NISP Slitless Spectroscopy" (2604.02726) describes the first large-scale spectroscopic characterization of galaxy-scale strong gravitational lenses in Euclid's Quick Data Release 1 (Q1), using the Near-Infrared Spectrometer and Photometer (NISP) slitless spectroscopy. The authors provide secure redshifts for both the foreground deflector and background source for 178 systems, leveraging anti-parallel dual-grism orientations for independent confirmation and achieving a sample size that surpasses all previous targeted spectroscopic lens surveys. The study outlines methodology, quality assurance, astrophysical insights, and the scalable implications for Euclid's wide survey.

Methodology

Input Data and Coverage

A total of 579 strong lens candidates identified by the Euclid Q1 Strong Lensing Discovery Engine in 63 deg$^2$ (with ensemble morphological classification and expert visual confirmation) were analyzed. Of these, 473 objects fell within the footprint of the NISP SIR spectroscopic coverage (1.25–1.85 $\mu$m, R$\,\sim\,$450), including both primary and supplementary catalogs.

Extraction and Line Identification

Spectra were optimally extracted using a spatial aperture centered on the candidate, leveraging inverse variance weighting, and underwent robust background subtraction. Emission and absorption features were detected via continuum removal and automated peak detection, followed by Gaussian fitting for centroid refinement. Emission line redshifts were determined from matching three or more nebular lines ([O\,II], H$\beta$, [O\,III], H$\alpha$, [S\,II]) within the NISP bandpass, while deflector redshifts relied on two or more prominent absorption features (Ca\,II triplet, Mg\,I\,b, Na\,D at $0.4 \lesssim z \lesssim 1.2$, and blue features for $z \gtrsim 2$).

Dual-Grism Independent Confirmation

The dual anti-parallel RGS000/RGS180 grism exposures serve as an internal quality benchmark against contamination and systematic errors. Cross-correlation criteria—requiring a correlation coefficient $r>0.3$ and $\Delta\lambda<50$\,\AA—yielded a $94\%$ confirmation rate for sources with dual-grism coverage.

Results

Catalog Statistics and Quality Tiers

Of the 473 covered systems, 461 yielded reliable spectroscopic measurements. The sample includes:

• 419 secure source redshifts ($\mu$03 emission lines, SNR$\mu$110)
• 199 secure deflector redshifts ($\mu$22 absorption features)
• 178 with complete ($\mu$3, $\mu$4) pairs
• 148 gold-complete systems (dual-grism confirmed, both $\mu$5 and $\mu$6)

The catalog is stratified into gold-complete, gold-source, silver, and bronze tiers, reflecting the robustness of spectroscopic identification and dual-grism confirmation.

Redshift Distributions

Source redshifts span $\mu$7 (median 1.59), while deflector redshifts range $\mu$8 (median 1.06). The emission line census facilitates dust correction and SFR estimates for $\mu$9 of sources (both H$\,\sim\,$0 and H$\,\sim\,$1 detected). [O\,III] and H$\,\sim\,$2 are most frequently detected, consistent with the NISP bandpass.

Astrophysical Structure

Redshift and spatial clustering reveal group and filament environments purely via spectroscopic lensing data, without auxiliary photometric or X-ray/SZ data. Deflector overdensities at $\,\sim\,$3 and $\,\sim\,$4, and a source-plane spike at $\,\sim\,$5 (3.1$\,\sim\,$6 above baseline), demonstrate lens selection as an effective probe of large-scale structure.

Ten systems with $\,\sim\,$7 are flagged as projected pairs or misidentified geometry.

Comparative Survey Analysis

This campaign shows several advantages over prior targeted lens surveys:

• Largest spectroscopic lens sample: Exceeds SLACS, BELLS, SL2S, AGEL, and GLASS in individual sample size.
• Zero dedicated follow-up: All spectra acquired as part of standard Euclid survey operations.
• Wider redshift range and NIR access: Probes redshifts beyond the capability of SDSS and ground-based optical programs, with 25 deflectors at $\,\sim\,$8.
• Dual-grism verification: Provides superior internal confirmation, mitigating slitless contamination, which prior fiber/slit surveys lack.

Direct comparison shows that this sample’s 178 complete redshift pairs already outnumber the combined totals of all previous lens surveys.

Practical and Theoretical Implications

Mass Modeling and Cosmography

The 178 complete ($\,\sim\,$9, $\beta$0) pairs, and especially the 148 gold-complete, provide the required inputs for measuring enclosed lens masses via angular Einstein radii and $\beta$1, enabling studies of the density profile evolution of galaxies to $\beta$2. The sample, once externally validated and paired with precise lens models, enables lens statistics-based cosmography, with potential sub-percent precision on $\beta$3 upon scaling to the Euclid wide survey.

Source and Deflector Physical Diagnostics

Multiple emission line detections allow for dust-corrected SFR, metallicity diagnostics ([N\,II]/H$\beta$4 ratios for N2 index), and mass-metallicity studies for sub-$\beta$5 galaxies at high redshift.

Group and Structure Identification

Spectroscopic clustering in deflector and source redshifts reaffirms lensing surveys as independent group and filament finders at $\beta$6, complementing traditional approaches.

Scaling to Euclid Wide Survey

Extrapolation suggests that NISP slitless spectroscopy will yield redshifts for $\beta$7100,000 galaxy-scale lenses and $\beta$830,000 complete pairs across 14,500\,deg$\beta$9, an expansion of three orders of magnitude over current samples. Improved depth and exposure stacking will raise the dual-grism confirmation rate and redshift reach.

Caveats and Validation

Pending independent redshift validation from official Euclid pipelines, direct comparison with ground-based and targeted spectroscopy is necessary. The completeness is limited to emission-line sources ($\alpha$072\%). Spectral deblending and wavelength calibration biases require further assessment, and users should apply caution when using systems below gold tier.

Conclusion

This work represents the largest spectroscopic characterization of galaxy-scale strong gravitational lenses to date, leveraging survey-mode NISP slitless spectroscopy and dual-grism confirmation to achieve superior robustness and extended redshift coverage. The publicly available quality-tiered catalog of 461 systems enables mass modeling, lens statistics, and population studies with unprecedented scale and efficiency. Scaling to the full Euclid survey will transform lens-based cosmography and extragalactic astrophysics, offering a major new resource for studies of galaxy evolution, large-scale structure, and cosmological parameters.