Confirming HSC strong lens candidates with DESI Spectroscopy. I. Project Overview

Abstract: Accurate redshift determinations of both lenses and sources are critical for confirming strong-lens systems and fully realizing their scientific value. However, the thousands of strong-lens candidates now routinely discovered in wide-field imaging surveys make one-by-one follow-up observations impractical. In this work, we investigate the capability and efficiency of large-scale spectroscopic surveys in confirming strong-lens systems. As a case study, we cross-match strong lens candidates identified from the Hyper Suprime-Cam Subaru Strategic Program with Data Release 1 of the Dark Energy Spectroscopic Instrument (DESI). We find that DESI DR1 serendipitously observed putative lenses and/or lensed images in approximately 50\% of these candidates. Analyzing the DESI spectra for $\approx 500$ matched candidates that meet our specific interests, we determine both lens and source redshifts for 27 systems. Additionally, 76 candidate systems feature lensing galaxies at $z > 0.8$, and one candidate system contains a quasar within its lensing galaxy. Applying this approach to other strong-lens candidates will yield numerous additional confirmations, with a further several-fold increase anticipated from the final DESI data release. Our results highlight the growing importance of large-scale spectroscopic surveys in advancing strong lensing discoveries and investigations.