Investigating the growth of little red dot descendants at z < 4 with the JWST

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM 91191 Gif-sur-Yvette, France
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio via Gobetti 93/3 40129 Bologna, Italy
³ Racah Institute of Physics, The Hebrew University Jerusalem 91904, Israel
⁴ Center for Astrophysics | Harvard & Smithsonian 60 Garden St Cambridge MA 02138, USA
⁵ Black Hole Initiative, Harvard University 20 Garden St Cambridge MA 02138, USA
⁶ NSF’s National Optical-Infrared Astronomy Research Laboratory 950 North Cherry Avenue Tucson AZ 85719, USA
⁷ University of Massachusetts Amherst 710 North Pleasant Street Amherst MA 01003-9305, USA
⁸ Department of Physics and Astronomy, University of Louisville Natural Science Building 102 40292 KY Louisville, USA
⁹ Department of Physics and Astronomy, Colby College Waterville ME 04901, USA
¹⁰ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
¹¹ Department of Astronomy, The University of Texas at Austin Austin TX 78712, USA
¹² Centro de Astrobiología (CAB), CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 July 2025
Accepted: 15 November 2025

Abstract

Context. One of the most remarkable and unexpected results of the James Webb Space Telescope is the discovery of a population of compact red galaxies: the so-called little red dots (LRDs). The existence of these galaxies raises many questions, including that of their nature and origin, as well as that of their evolution. Indeed, these compact red sources exhibit a pronounced decline in number density by nearly two orders of magnitude from z = 6 to z = 3.

Aims. In this paper, we investigate the possible evolution of this galaxy population at a lower redshift. To this end, we have identified a sample of candidates in the CEERS images that could represent the descendants of LRDs by assuming a single evolutionary path: the development of a blue star-forming outskirt while retaining a inner red core.

Methods. Our color–magnitude selection identifies red galaxies as red as LRDs at z < 4, defined by a compact, red, inner region and blue outskirts. The red core is associated with the LRD population, while the blue periphery traces recently formed young stars. Morphological properties were derived by fitting single Sérsic profiles, while other physical quantities were obtained through spectral energy distribution (SED) fitting, assuming a stellar-only model for both the inner region and the outskirts.

Results. The selected galaxies are likely “post-LRDs” galaxies, showing similar properties to LRDs under a stellar-only model: stellar masses of M_* ≈ 10¹⁰ M_⊙, central densities Σ_* ≈ 10¹¹ M_⊙ kpc⁻², similar rest-frame red colors, and a ∼1 kpc offset below the size–mass relation. Their number density at z = 3 ± 0.5 (10^−4.15 Mpc⁻³) matches that of LRDs at 5 < z < 7, supporting an evolutionary connection. We find a strong redshift-dependent increase in both outskirts’ mass fraction and galaxy size, from ∼250 pc at z = 5 to ∼600 pc at z = 3, indicating overall stellar growth. Meanwhile, the core remains as red and as massive, but the characteristic V shaped SED fades as the extended star-forming envelope becomes dominant.

Conclusions. These findings support an evolutionary scenario in which LRDs gradually acquire an extended stellar component over cosmic time by cold accretion. This growth affects the initial LRD state in two key ways: the physical size increases and the characteristic V shaped SED in the core becomes less distinct and disappears. As a result, the original selection criteria based on both of them can no longer identify this population as it evolves, providing an explanation for their observed decline in number density.