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

Abstract

AbstractOne 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 $${z}_{{{{\rm{phot}}}}}=1.9{4}_{-0.17}^{+0.13}$$ 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 $${z}_{{{{\rm{phot}}}}}=2.9{8}_{-0.47}^{+0.42}$$ 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 $${M}_{{{{\rm{lens}}}}}=6.{5}_{-1.5}^{+3.7}\times 1{0}^{11}$$ 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 $${M}_{{{{\rm{stars}}}}}=1.{1}_{-0.3}^{+0.2}\times 1{0}^{11}$$ 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 $${M}_{{{{\rm{dm}}}}}=2.{6}_{-0.7}^{+1.6}\times 1{0}^{11}$$ 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.