Euclid: A complete Einstein ring in NGC 6505

In this work, we reported the discovery of a complete Einstein ring around the elliptical galaxy NGC 6505, at z=0.042. This is the first strong gravitational lens discovered in Euclid and the first in an NGC object from any survey. The combination of the low redshift of the lens galaxy, the brightness of the source galaxy, and the completeness of the ring make this an exceptionally rare strong lens, unidentified until its observation by Euclid.

Figure 1: Euclid imaging data used in this work, and in which Altieri’s lens was discovered. The main panel shows a composite false colour image produced by combining the VIS and NISP data. The higher resolution broadband VIS IE image is used to set the brightness, with the colour provided by the lower resolution NISP YE, JE, and HE passband images. The central light of the galaxy is suppressed to make the lensed arc more visible. The inset shows only the higher resolution VIS data in the central 8′′ of the image, indicated by the square in the main panel. The angular scale and the physical scale at the redshift of the lens are given in each frame.

A first analysis on this magnificent source revealed an Einstein radius of 2.5 arcsec, corresponding to 2.1kpc at the redshift of the lens. This is small compared to the effective radius of the galaxy, Reff∼12.3arcsec. Combining the strong lensing measurements with analysis of spectroscopic data we estimated a dark matter fraction inside the Einstein radius of fDM=(11.1+5.4−3.5)% and a stellar initial mass-function (IMF) mismatch parameter of αIMF=1.26+0.05−0.08, indicating a heavier-than-Chabrier IMF in the centre of the galaxy.

If you are interested learning more about this amazing lens, then please have a read of this paper Euclid Collaboration : C. M. O’Riordan, L. J. Oldham, A. Nersesian, T. Li, et al. 2025, A&A, 694, A145 or have a look at the press release by ESA by clicking here.