Evolution of the Sérsic index up to z = 2.5 from JWST and HST

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
² Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109–1107, USA
³ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København N DK-2200, Denmark
⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁵ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
⁶ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
⁷ STAR Institute, Université de Liège, Quartier Agora, Allée du six Aout 19c, B-4000 Liège, Belgium
⁸ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁹ Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
¹⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹¹ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

^⋆ Corresponding author; marco.martorano@ugent.be

Received: 7 November 2024
Accepted: 3 January 2025

Abstract

Context. The Hubble Space Telescope (HST) has long been the only instrument able to allow us to investigate the structure of galaxies up to redshift z = 3, limited to the rest-frame UV and optical. The James Webb Space Telescope (JWST) is now unveiling the rest-frame near-IR structure of galaxies, less affected by dust attenuation and more representative of their underlying stellar mass profiles.

Aims. We measure the evolution with redshift of the rest-frame optical and near-IR Sérsic index (n), and examine the dependence on stellar mass and star-formation activity across the redshift range 0.5 ≤ z ≤ 2.5.

Methods. For an HST-selected parent sample in the CANDELS fields we infer rest-frame near-IR Sérsic profiles for ≈15 000 galaxies in publicly available NIRCam imaging mosaics from the COSMOS-Web and PRIMER surveys. We augment these with rest-frame optical Sérsic indices, previously measured from HST imaging mosaics.

Results. The median Sérsic index evolves slowly or not at all with redshift, except for very high-mass galaxies (M_⋆ > 10¹¹ M_⊙), which show an increase from n ≈ 2.5 to n ≈ 4 at z < 1. High-mass galaxies have higher n than lower-mass galaxies (the sample reaches down to M_⋆ = 10^9.5 M_⊙) at all redshifts, with a stronger dependence in the rest-frame near-IR than in the rest-frame optical at z > 1. This wavelength dependence is caused by star-forming galaxies that have lower optical than near-IR n at z > 1 (but not at z < 1). Both at optical and near-IR wavelengths, star-forming galaxies have lower n than quiescent galaxies, confirming and fortifying the result that across cosmic time a connection exists between star-formation activity and the radial stellar mass distribution. Besides these general trends that confirm previous results, two new trends emerge: (1) at z > 1 the median near-IR n varies strongly with star formation activity, but not with stellar mass, and (2) the scatter in near-IR n is substantially higher in the green valley (0.25 dex) than on the star-forming sequence and among quiescent galaxies (0.18 dex) – this trend is not seen in the optical because dust and young stars contribute to the variety in optical light profiles. Our newly measured rest-frame near-IR radial light profiles motivate future comparisons with radial stellar mass profiles of simulated galaxies as a stringent constraint on processes that govern galaxy formation.