2023/09/14 by Pieter van Dokkum, Gabriel Brammer, van Dokkum, Pieter +7 · 2 voices · 3 citations
Physics and Astronomy · #Adaptive optics and wavefront sensing #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena
paper · pdf · doi:10.48550/arxiv.2309.07969
openalex publication_date 2023/09/14 · openalex created_date 2023/09/19 · openalex updated_date 2026/07/28
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 × 1011 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 × 1011 Msun for a Chabrier initial mass function (IMF), and the fiducial dark matter mass is M\rm dm = 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.