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

TL;DR

This paper reports the discovery of a massive, compact, quiescent galaxy at z=2 with a complete Einstein ring observed by JWST, providing direct measurements of its core mass and insights into galaxy evolution.

Contribution

It presents the first direct measurement of the core mass of a massive galaxy at z=2 using a complete Einstein ring observed by JWST, offering new constraints on galaxy formation models.

Findings

Mass of the galaxy's core is approximately 6.5 x 10^11 Msun.

The stellar mass within 6.6 kpc is about 1.1 x 10^11 Msun.

Additional mass components are needed to explain lensing, suggesting complex dark matter or IMF properties.

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 $z_{\rm phot}=1.94^{+0.13}_{-0.17}$ with a complete Einstein ring. The ring was found in the JWST COSMOS-Web survey and is produced by a background galaxy at $z_{\rm phot}=2.98^{+0.42}_{-0.47}$. 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 $M_{\rm lens}=6.5^{+3.7}_{-1.5} \times 10^{11}$ Msun within a radius of 6.6 kpc. The stellar mass within the same radius is $M_{\rm stars}= 1.1^{+0.2}_{-0.3} \times 10^{11}$ Msun for a Chabrier initial mass function (IMF), and the fiducial dark matter mass is $M_{\rm dm} = 2.6^{+1.6}_{-0.7} \times 10^{11}$ 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.