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\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${z}_{{{{\\rm{phot}}}}}=1.9{4}_{-0.17}^{+0.13}$$\\end{document} 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\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${z}_{{{{\\rm{phot}}}}}=2.9{8}_{-0.47}^{+0.42}$$\\end{document}. 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\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{{{{\\rm{lens}}}}}=6.{5}_{-1.5}^{+3.7}\ imes 1{0}^{11}$$\\end{document} M⊙ within a radius of 6.6 kpc. The stellar mass within the same radius is Mstars=1.1−0.3+0.2×1011\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{{{{\\rm{stars}}}}}=1.{1}_{-0.3}^{+0.2}\ imes 1{0}^{11}$$\\end{document} M⊙ for a Chabrier initial mass function and the fiducial dark matter mass is Mdm=2.6−0.7+1.6×1011\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${M}_{{{{\\rm{dm}}}}}=2.{6}_{-0.7}^{+1.6}\ imes 1{0}^{11}$$\\end{document} 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.