Euclid Quick Data Release (Q1): XXIX. The strong-lensing discovery engine D: Double-source-plane lens candidates

Strong gravitational-lensing systems with multiple source planes are powerful tools for probing the density profiles and dark matter substructure of galaxies. The ratio of the Einstein radii is related to the dark energy equation of state through the cosmological scaling factor β. Galaxy-scale double-source-plane lenses (DSPLs) are extremely rare, however. We report the discovery of four new galaxy-scale DSPL candidates in the Euclid Quick Release 1 (Q1) data. These systems were initially identified through a combination of machine-learning lens-finding models and subsequent visual inspection from citizens and experts. We applied the widel -used LensPop lens-forecasting model to predict that the full Euclid survey will discover 1700 DSPLs. This scales to 6 ± 3 DSPLs in 63 deg2, which is the area of Q1. The number of discoveries in this work is broadly consistent with this forecast. We present lens models for each DSPL and infer their β values. Our initial Q1 sample demonstrates that Euclid promises to discover these rare objects.