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One of the surprising results from HST was the discovery that many of themost massive galaxies at z~2 are very compact, having half-light radii of only1-2 kpc. The interpretation is that massive galaxies formed inside-out, withtheir cores largely in place by z~2 and approximately half of their present-daymass added later through minor mergers. Here we present a compact, massive,quiescent galaxy at $z_{\rm phot}=1.94^{+0.13}_{-0.17}$ with a completeEinstein ring. The ring was found in the JWST COSMOS-Web survey and is producedby a background galaxy at $z_{\rm phot}=2.98^{+0.42}_{-0.47}$. Its 1.54"diameter provides a direct measurement of the mass of the "pristine" core of amassive galaxy, observed before mixing and dilution of its stellar populationduring the 10 Gyr of galaxy evolution between z=2 and z=0. We find a mass of$M_{\rm lens}=6.5^{+3.7}_{-1.5} \times 10^{11}$ Msun within a radius of 6.6kpc. The stellar mass within the same radius is $M_{\rm stars}=1.1^{+0.2}_{-0.3} \times 10^{11}$ Msun for a Chabrier initial mass function(IMF), and the fiducial dark matter mass is $M_{\rm dm} = 2.6^{+1.6}_{-0.7}\times 10^{11}$ Msun. Additional mass is needed to explain the lensing results,either in the form of a higher-than-expected dark matter density or abottom-heavy IMF.

A massive compact quiescent galaxy at z=2 with a complete Einstein ring in JWST imaging