Sizes and Mass Profiles of Candidate Massive Galaxies Discovered by JWST at 7 < z < 9: Evidence for Very Early Formation of the Central ∼100 pc of Present-day Ellipticals

Abstract

The first JWST data revealed an unexpected population of red galaxies that appear to have redshifts of z ∼ 7–9 and high masses of M * ≳ 1010 M ☉. Here we fit Sérsic profiles to the F200W NIRCam images of the 13 massive galaxy candidates of Labbé et al., to determine their structural parameters. Satisfactory fits were obtained for nine galaxies. We find that their effective radii are extremely small, ranging from r e ∼ 80 pc to r e ∼ 300 pc, with a mean of 〈r e〉 ≈ 150 pc. For their apparent stellar masses, the galaxies are smaller than any other galaxy population that has been observed at any other redshift. We use the fits to derive circularized three-dimensional stellar mass profiles of the galaxies, and compare these to the mass profiles of massive quiescent galaxies at z ∼ 2.3 and nearby elliptical galaxies. Despite the fact that the high-redshift galaxies have 10–20 times smaller half-light radii than their putative descendants, the central stellar densities are very similar. The most straightforward interpretation is that the dense compact inner regions of the most massive ellipticals today were already in place ∼600 Myr after the Big Bang. We caution that the redshifts and masses of the galaxies remain to be confirmed, and that the complex NIRCam point-spread function is not yet fully characterized.