Investigating the Growth of Little Red Dot Descendants at z<4 with the JWST

Jean-Baptiste Billand, David Elbaz, Fabrizio Gentile, Maxime Tarrasse, Maximilien Franco, Benjamin Magnelli, Emanuele Daddi, Yipeng Lyu, Avishai Dekel, Fabio Pacucci, Valentina Sangalli, Mark Dickinson, Mauro Giavalisco, Benne W. Holwerda, Dale D. Kocevski, Anton M. Koekemoer, Vasily Kokorev, Ray A. Lucas, Pablo G. Pérez-González

First listed 2025-07-05 | Last updated 2025-11-19

Abstract

One of JWST's most remarkable discoveries is a population of compact red galaxies known as Little Red Dots (LRDs). Their existence raises many questions about their nature, origin, and evolution. These galaxies show a steep decline in number density-nearly two orders of magnitude-from $z=6$ to $z=3$. In this study, we explore their potential evolution by identifying candidate descendants in CEERS, assuming a single evolutionary path: the development of a blue star-forming outskirt around the red compact core. Our color-magnitude selection identifies galaxies as red as LRDs at $z<4$, surrounded by young, blue stellar outskirts. Morphological parameters were derived from single Sérsic profile fits; physical properties were obtained from SED fitting using a stellar-only model. These "post-LRD" candidates show LRD-like features with $M_\ast \sim 10^{10} \ M_\odot $, central densities ($ Σ_\ast \sim 10^{11} \ M_\odot \ \text{kpc}^{-2}$ ), compact sizes, and red rest-frame colors, but with an added extended component. Their number density at $z = 3 \pm 0.5$ ( $ \sim 10^{-4.15} \, \text{Mpc}^{-3} $) matches that of LRDs at $5 < z < 7$ , supporting a possible evolutionary link. We observe a redshift-dependent increase in outskirts mass fraction and galaxy size-from $\sim 250$ pc at $ z = 5 $ to $\sim 600$ pc at $ z = 3 $-suggesting global stellar growth. Meanwhile, the core remains red and compact, but the V-shaped SED fades as the outskirts grow. These findings support an evolutionary scenario in which LRDs gradually acquire an extended stellar component over cosmic time by cold accretion. This may explain the apparent decline in their observed number density at lower redshift.

Short digest

Using CEERS imaging, the authors select z<4 “post-LRD” candidates by requiring a compact, very red core plus a blue star-forming envelope, then model inner/outskirts with stellar-only SED fits and single-Sérsic morphologies. The sample shows LRD-like cores with M* ~1e10 Msun, Σ* ~1e11 Msun kpc^-2, and compact sizes about 1 kpc below the size–mass relation, while adding an extended young component; their number density at z=3±0.5 (~10^-4.15 Mpc^-3) matches that of LRDs at 5<z<7, indicating an evolutionary link. Outskirts mass fraction and size increase toward lower redshift (≈250 pc at z=5 to ≈600 pc at z=3), as the core stays red/compact and the hallmark V-shaped SED fades, consistent with growth by cold accretion. This offers an explanation for the apparent decline of LRD counts at lower z, with the caveat that it assumes a single evolutionary path and stellar-only modeling.

Key figures to inspect

• Figure 1: Inspect where the color–magnitude–selected post-LRDs fall relative to the CEERS locus and the F277W–F444W > 1.5 LRD red-core cut; verify that selected sources are LRD-red in the core while permitting blue outskirts.
• Figure 2: Check the segmentation/ellipse defining inner vs outskirts and compare to the F444W PSF circle to confirm the red core is resolved and the blue envelope extends beyond the beam.
• Figure 3: Examine the redshift distribution and the six objects overlapping the Kocevski et al. (2025) LRD sample; use this to contextualize the z=3±0.5 bin used for the number-density comparison.
• Figure 4: Use the mock-injection errors vs half-light radius and Sérsic n, along with the F444W HWHM and adopted resolution limit, to gauge the robustness of compactness claims and any size floor biases.