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

Abstract

One of the surprising results from HST was the discovery that many of the most massive galaxies at z~2 are very compact, having half-light radii 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 $z_{\mathrm{phot}}=1.94_{-0.17}^{+0.13}$ with a complete Einstein ring. The ring was found in the JWST COSMOS-Web survey and is produced by a background galaxy at $z_{\mathrm{phot}}=2.98_{-0.47}^{+0.42}$. Its 1.54" diameter provides a direct measurement of the mass of the "pristine" core of a massive galaxy, observed before mixing and dilution of its stellar population during the 10 Gyr of galaxy evolution between z=2 and z=0. We find a mass of $M_{\mathrm{lens}}=6.5_{-1.5}^{+3.7}\times 10^{11}$ Msun within a radius of 6.6 kpc. The stellar mass within the same radius is $M_{\mathrm{stars}}=1.1_{-0.3}^{+0.2}\times 10^{11}$ Msun for a Chabrier initial mass function (IMF), and the fiducial dark matter mass is $M_{\mathrm{dm}}=2.6_{-0.7}^{+1.6}\times 10^{11}$ Msun. Additional mass is needed to explain the lensing results, either in the form of a higher-than-expected dark matter density or a bottom-heavy IMF.

Nature Astronomy, in press. As it happens this is the highest redshift lens currently known. Found by eyeballing the wonderful COSMOS-Web data