Testing the recovery of the NGC 6505 Einstein ring in deep OSN 1.5-m imaging
arXiv:2607.12209
The paper evaluates whether a deep 1.5‑m ground‑based R‑band image can recover the Einstein ring of the low‑redshift galaxy NGC 6505 discovered by Euclid, and finds that the ring is not detectable because the seeing is comparable to the ring’s angular size.
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.
Submitted to Astronomy and Astrophysics. 5 figures, 2 tables, 7 pages