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 Euclid mission provides a rare, precisely characterised low-redshift strong lens. Its Einstein radius of 2.500 arcsec 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 test 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 of FWHM = 2.47 arcsec, corresponding to an Einstein-radius-to-FWHM ratio of 1.01. After subtracting a non-parametric isophotal model of the galaxy, we search the residual for coherent arc- or ring-like structure and find none at the Euclid-measured Einstein radius, with a maximum value of 1.5 sigma for the adopted radial recovery metric. An injection-recovery experiment using a simplified, Euclid-informed synthetic ring, processed through the same isophotal-modelling and residual-analysis pipeline, yields a maximum recovered significance between 0.42 sigma and 1.86 sigma across three PSF descriptions, below our operational 3 sigma recovery threshold in every case. We interpret this as a dataset-specific non-recovery driven by the regime in which the Einstein radius is approximately equal to the FWHM of these observations, rather than as a general detectability law.