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

TL;DR

This study analyzes the sizes and mass profiles of candidate massive galaxies at redshifts 7-9 discovered by JWST, revealing extremely compact structures with dense cores that suggest early formation of elliptical galaxy centers.

Contribution

It provides the first detailed structural analysis of high-redshift massive galaxy candidates, showing their remarkably small sizes and dense cores compared to later galaxy populations.

Findings

Galaxies have effective radii of 80-300 pc, much smaller than other observed populations.

Central stellar densities are comparable to those of present-day ellipticals.

Dense inner regions of massive ellipticals were already in place 600 million years after the Big Bang.

Abstract

The first JWST data revealed an unexpected population of red galaxies that appear to have redshifts of $z\sim 7-9$ and high masses of $M_*$ $\sim$ 10$^{10}$ M$_{\odot}$ (Labb\'e et al. 2023). Here we fit S\'ersic profiles to the F200W NIRCam images of the 13 massive galaxy candidates of Labb\'e 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 $r_{\rm e}\sim 80$ pc to $r_{\rm e} \sim 300$ pc, with a mean of $\langle r_{\rm e}\rangle \approx 150$ 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\sim$2.3 and nearby elliptical galaxies. We find that, despite the high redshift galaxies having $10-20$ times smaller half-light radii, the central stellar densities are comparable to those of their putative descendants at later times. The most straightforward interpretation is that the dense compact inner regions of the most massive ellipticals today were already in place $\sim 600$ 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.