The DESI Single Fiber Lens Search. I. Four Thousand Spectroscopically Selected Galaxy-Galaxy Gravitational Lens Candidates

TL;DR

This work presents a large spectroscopic search for galaxy-galaxy strong lenses using DESI data, identifying 4,110 strong lens candidates (3,887 new) by detecting background [O II] emission in foreground LRG spectra. The authors implement a three-phase pipeline—foreground subtraction with Redrock, a linear residual search for a double-Gaussian [O II] profile, and a constrained MCMC refinement—followed by rigorous quality cuts to isolate genuine lens signals. They quantify lensing likelihood from redshift and flux information and demonstrate the method’s potential to build a cosmological lens dataset, with applications to time-delay cosmography and dark matter substructure studies, including independently recovering the host of the lensed supernova iPTF16geu. The resulting catalog provides a rich resource for high-resolution follow-up imaging and monitoring for multiply lensed transients, enabling precise measurements of $H_0$ and substructure constraints at cosmological distances.

Abstract

We present 4,110 strong gravitational lens candidates, 3,887 of which are new discoveries, selected from a sample of 5,837,154 luminous red galaxies (LRGs) observed with the Dark Energy Spectroscopic Instrument (DESI). Candidates are identified via the presence of background ionized oxygen [O II] nebular emission lines in the foreground LRG spectra which may originate from the lensing of higher redshift star-forming galaxies. Using the measured foreground redshift, background redshift, and integrated flux of the background [O II] doublet, we integrate over impact parameters to compute the probability that each candidate is a lens. We expect 53% of candidates to be true lenses with Einstein radii ranging from 0.1'' to 4'', which can be confirmed with high-resolution imaging. Confirmed strong lenses from this sample will form a valuable cosmological dataset, as strong gravitational lensing is the only method to directly measure dark matter halo substructure at cosmological distances. We independently recover the host of the multiply imaged gravitationally lensed type Ia supernova iPTF16geu (Goobar et al. 2017). Monitoring these lenses for future multiply lensed transients will enable a) H0 measurements via time-delay cosmography and b) substructure measurements via flux ratios.

Figures (2)

• Figure 1: Distribution of luminous red galaxy (LRG) redshifts ($z_{\text{LRG}}$) observed in dark time included in the DESI Data Release 2 (DR2). Densities are calculated using a Gaussian density kernel estimator with a bandwidth factor of 0.1. We search this sample of 5,837,154 spectra for lenses.
• Figure 2: Distribution of foreground ( Redrock best fit) and background (determined by the secondary [O ii] doublet) redshifts for lens candidates. The solid black line indicates where $z_{\text{FG}} = z_{\text{BG}}$. Spectra of interest remaining after the secondary MCMC fit and cuts are applied (end of Phase 2). Several linear streak-like features are apparent in the $z_{\text{BG}}$ vs. $z_{\text{FG}}$ distribution. These contaminants are largely due to rare emission lines in the foreground LRG spectrum which are badly fit by Redrock, as indicated by their colors and labels. Sources with $z_{\text{FG}} < 0.005$ are likely M stars blended with faint galaxies. We remove these contaminants (Phase 3). The remaining 4,110 sources plotted here form the lens candidate catalog (dark blue). The side panels show histogram distributions of the 4,110 candidates' foreground and background redshifts.