James Webb Space Telescope Feed Post

A massive compact quiescent galaxy at z=2 with a complete Einstein ring in JWST imaging (COSMOS-Web)

Symmetries in the ring. This color image was created from the F150W and F444W images, after subtracting the galfit model of the central galaxy. The image spans 2.5 '' × 2.5 ''. There are several candidate multiply-imaged features along the ring. The two red knots are very bright in F444W and are stretched into mirrored arcs. This would be difficult to explain by any other mechanism than gravitational lensing. 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 zphot=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 zphot=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 Mlens=6.5+3.7-1.5×1011 Msun within a radius of 6.6 kpc. The stellar mass within the same radius is Mstars=1.1+0.2-0.3×1011 Msun for a Chabrier initial mass function (IMF), and the fiducial dark matter mass is Mdm=2.6+1.6-0.7×1011 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.