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

One of the surprising results from the Hubble Space Telescope was the discovery that many of the most massive galaxies at redshift z ≈ 2 are very compact, having a half-light radius of only 1-2 kpc. The interpretation is that massive galaxies formed inside out, with their cores largely in place by z ≈ 2 and approximately half of their present-day mass added later through minor mergers. Here we present a compact, massive, quiescent galaxy at a photometric redshift of $z_{\mathrm{phot}}=1.94_{-0.17}^{+0.13}$ with a complete Einstein ring. The ring was found in the James Webb Space Telescope COSMOS-Web survey and is produced by a background galaxy at $z_{\mathrm{phot}}=2.98_{-0.47}^{+0.42}$. Its 1.54″ diameter provides a direct measurement of the mass of the 'pristine' core of a massive galaxy, observed before the mixing and dilution of its stellar population during the 10 Gyr of galaxy evolution between z = 2 and z = 0. We find a mass for the lens $M_{\mathrm{lens}}=6.5_{-1.5}^{+3.7}\times 10^{11}$ M_⊙ within a radius of 6.6 kpc. The stellar mass within the same radius is $M_{\mathrm{stars}}=1.1_{-0.3}^{+0.2}\times 10^{11}$ M_⊙ for a Chabrier initial mass function and the fiducial dark matter mass is $M_{\mathrm{dm}}=2.6_{-0.7}^{+1.6}\times 10^{11}$ M_⊙. Additional mass appears to be needed to explain the lensing results, either in the form of a higher-than-expected dark matter density or a bottom-heavy initial mass function.