The COSMOS-Web Lens Survey(COWLS) III: forecasts versus data

Abstract

We compare forecasts for the abundance and properties of strong gravitational lenses in the COSMOS-Web survey, a 0.54 deg2 survey of the COSMOS field using the NIRCam and MIRI instruments aboard James Webb Space Telescope (JWST), with the first catalogue of strong lens candidates identified in the observed NIRCam data, COWLS. We modify the lenspop package to produce a forecast for strong lensing in COSMOS-Web. We add a new mock galaxy catalogue to use as the source population, as well as the COSMOS-Web survey specifications, including the transmission data for the four NIRCam filters used. We forecast 107 strong lenses can be detected in COSMOS-Web across all bands, assuming complete subtraction of the lens galaxy light. The majority of the lenses are forecast to have small Einstein radii (θE < 1$ arcsec) and lie at redshifts in the range 0 < z < 2, while the source redshift distribution peaks at z ~ 3 and has a long tail extending up to z ~ 11, unambiguously showing that strong lensing in JWST can probe the entirety of the epoch of reionization. We compare our forecast with the distributions of Einstein radii, lens photometric redshifts, and lens and source magnitudes in the observed lenses, finding that while the forecast and observed Einstein radii distributions match, the redshifts and magnitudes do not. The observed lens redshift distribution peaks at a slightly lower redshift than the forecast one, while the lens magnitudes are systematically brighter in the observed data than in the forecast.