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

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 zphot=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 zphot=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 Mlens=6.5‑1.5+3.7×1011 M_⊙ within a radius of 6.6 kpc. The stellar mass within the same radius is Mstars=1.1‑0.3+0.2×1011 M_⊙ for a Chabrier initial mass function and the fiducial dark matter mass is Mdm=2.6‑0.7+1.6×1011 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.