Testing the recovery of the NGC 6505 Einstein ring in deep OSN 1.5 m imaging

Abstract

The discovery of a complete Einstein ring around the nearby elliptical galaxy NGC 6505 (z = 0.042) by the mission provides a rare, precisely characterised low-redshift strong lens. Its Einstein radius, theta_ Euclid ̊m E = 2.500 ≈ regime of these observations and not as a general detectability law. is comparable to the angular resolution attainable from typical mid-aperture ground-based facilities, which makes the recoverability of the ring from the ground an open practical question. We tested whether the ring can be recovered in deep Cousins-R imaging of NGC 6505 obtained with the 1.5 m T150 telescope at the Sierra Nevada Observatory (OSN), totalling 8.03 h of integration at a final effective seeing with a full width at half maximum (FWHM) of 2.47 / = 1.01). After subtracting a non-parametric isophotal model of the galaxy, we searched the residual for coherent arc- or ring-like structure and found none at the -measured Einstein radius, with a maximum value of 1.5σ for the adopted radial recovery metric. An injection--recovery experiment using a simplified -informed synthetic ring, processed through the same isophotal-modelling and residual-analysis pipeline, yielded a maximum recovered significance between 0.42σ and 1.86σ across three point spread function descriptions, which is below our operational 3σ recovery threshold in every case. We interpret this as a dataset-specific non-recovery driven by the theta_ (theta_ ̊m E FWHM Euclid Euclid ̊m E FWHM