Euclid: A complete Einstein ring in NGC 6505

We report 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 ($I_\mathrm{E}=18.1$ lensed, $I_\mathrm{E}=21.3$ unlensed), and
the completeness of the ring make this an exceptionally rare strong lens,
unidentified until its observation by Euclid. We present deep imaging data of
the lens from the Euclid Visible Camera (VIS) and Near-Infrared Spectrometer
and Photometer (NISP) instruments, as well as resolved spectroscopy from the
Keck Cosmic Web Imager (KCWI). The Euclid imaging in particular presents one of
the highest signal-to-noise ratio optical/near-infrared observations of a
strong gravitational lens to date. From the KCWI data we measure a source
redshift of $z=0.406$. Using data from the Dark Energy Spectroscopic Instrument
(DESI) we measure a velocity dispersion for the lens galaxy of
$\sigma_\star=303\pm15\,\mathrm{kms}^{-1}$. We model the lens galaxy light in
detail, revealing angular structure that varies inside the Einstein ring. After
subtracting this light model from the VIS observation, we model the strongly
lensed images, finding an Einstein radius of 2.5 arcsec, corresponding to
$2.1\,\mathrm{kpc}$ at the redshift of the lens. This is small compared to the
effective radius of the galaxy, $R_\mathrm{eff}\sim 12.3\,\mathrm{arcsec}$.
Combining the strong lensing measurements with analysis of the spectroscopic
data we estimate a dark matter fraction inside the Einstein radius of
$f_\mathrm{DM} = (11.1_{-3.5}^{+5.4})\%$ and a stellar initial mass-function
(IMF) mismatch parameter of $\alpha_\mathrm{IMF} = 1.26_{-0.08}^{+0.05}$,
indicating a heavier-than-Chabrier IMF in the centre of the galaxy.