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Abstract 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{{₇₎ₓ}}=1. 94-₀. ₁₇^+0. 13 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{{₇₎ₓ}}=2. 98-₀. ₄₇^+0. 42 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{{₋₄₍ₒ}}=6. 5-₁. ₅^+3. 7 10^11 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{{ₒₓ₀ₑₒ}}=1. 1-₀. ₃^+0. 2 10^11 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{{₃₌}}=2. 6-₀. ₇^+1. 6 10^11 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.
Dokkum et al. (Thu,) studied this question.