Euclid: The Early Release Observations Lens Search Experiment

We investigate the ability of the Euclid telescope to detect galaxy-scale gravitational lenses. To do so, we perform a systematic visual inspection of the 0.7\,deg² Euclid ERO data towards the Perseus cluster using both the high-resolution VIS IE band, and the lower resolution NISP bands. We inspect every extended source brighter than magnitude $23$ in IE with $41$ expert human classifiers. This amounts to $12\,086$ stamps of 10^\,×\,10^. We find $3$ grade A and $13$ grade B candidates. We assess the validity of these $16$ candidates by modelling them and checking that they are consistent with a single source lensed by a plausible mass distribution. Five of the candidates pass this check, five others are rejected by the modelling and six are inconclusive. Extrapolating from the five successfully modelled candidates, we infer that the full 14\,000\, deg² of the Euclid Wide Survey should contain 100\,000+70\,000-30\,000 galaxy-galaxy lenses that are both discoverable through visual inspection and have valid lens models. This is consistent with theoretical forecasts of $170\,000$ discoverable galaxy-galaxy lenses in Euclid. Our five modelled lenses have Einstein radii in the range 0.\!\!^68\,<\,θE\,<1.\!\!^24, but their Einstein radius distribution is on the higher side when compared to theoretical forecasts. This suggests that our methodology is likely missing small Einstein radius systems. Whilst it is implausible to visually inspect the full Euclid data set, our results corroborate the promise that Euclid will ultimately deliver a sample of around 10⁵ galaxy-scale lenses.