James Webb Space Telescope Feed Post

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

Photometry of the lens and source. Top panels: images in the HST/ACS F814W band and the JWST/NIRCam F115W, F150W, F277W, and F444W bands. They are shown at a common F? scale. a) SED of the lens galaxy, determined from forced galfit fits. The galaxy is well-fit by a quiescent stellar population at z = 1.94+0.13 -0.17 and a total stellar mass of 1.1 +0.3 -0.4 ×1011 M? (for a Chabrier IMF). b) SED of the lens galaxy, with the summed flux of the two red knots shown in black and the blue ring in grey. The red knots provide a reasonably well-constrained redshift of zphot = 2.97+0.44 -0.37. Data are presented as measurements ± sd. 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.