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 phot = 1.9 4 - 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 phot = 2.9 8 - 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 lens = 6 . 5 - 1.5 + 3.7 × 1 0 11   M ⊙ within a radius of 6.6 kpc. The stellar mass within the same radius is M stars = 1 . 1 - 0.3 + 0.2 × 1 0 11   M ⊙ for a Chabrier initial mass function and the fiducial dark matter mass is M dm = 2 . 6 - 0.7 + 1.6 × 1 0 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.