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

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.

💡 Research Summary

The authors investigate the puzzling decline in the observed number density of “Little Red Dots” (LRDs) – a population of compact, red, high‑redshift galaxies discovered by JWST – from z ≈ 6 to z ≈ 3. LRDs are characterized by stellar masses of ≳10¹⁰ M☉, extremely high central stellar surface densities (Σ_* ≈ 10¹¹ M☉ kpc⁻²), and half‑light radii of only a few hundred parsecs. Their comoving number density drops by nearly two orders of magnitude between z = 6 and z = 3, suggesting that either they disappear or evolve into a different class that no longer satisfies the selection criteria used at higher redshift.

To test the hypothesis that LRDs evolve by acquiring a young, blue, star‑forming envelope while retaining their red, compact core, the authors search for “post‑LRD” candidates in the CEERS NIRCam fields (≈ 97 arcmin²) supplemented by HST/ACS imaging. Starting from the Merlin et al. (2024) multi‑band catalog (≈ 82 k objects), they apply a series of quality cuts (full filter coverage, S/N > 10 in F444W and F356W, F444W < 27 mag) to obtain a clean parent sample of 18 218 sources. A color–magnitude selection mimicking the LRD core (F277W – F444W > 1.5) combined with a blue outer‑region indicator (F150W – F356W < 0.5) isolates 263 objects with red cores. Visual inspection of the high‑resolution images and a pixel‑by‑pixel color analysis then reveal 55 galaxies that simultaneously display a compact red nucleus and an extended blue periphery.

Morphological parameters are derived by fitting a single Sérsic profile with GALFIT; the fits yield high Sérsic indices (n ≈ 4) and effective radii of 0.3–0.6 kpc, essentially identical to the original LRDs. To separate the core and envelope contributions, the authors perform two‑component SED fitting using FAST with BC03 stellar population models and a Chabrier IMF, assuming purely stellar emission (no AGN component). The total stellar masses cluster around 10¹⁰ M☉, and the central surface densities remain ≈10¹¹ M☉ kpc⁻². Crucially, the fraction of stellar mass residing in the blue envelope rises from ~20 % at z ≈ 5 to ~45 % at z ≈ 3, while the overall half‑light radius grows from ~250 pc to ~600 pc over the same interval. This trend indicates gradual, inside‑out growth of the galaxies.

The comoving number density of the 55 post‑LRD candidates, calculated via a V_max method, is 10⁻⁴·¹⁵ Mpc⁻³ at z = 3 ± 0.5, which matches the number density measured for LRDs at 5 < z < 7. This agreement supports a direct evolutionary link: LRDs at early times become the red‑core/blue‑envelope systems observed at lower redshift, and eventually evolve into more typical massive galaxies that no longer meet the original LRD selection criteria.

The authors interpret the observed envelope buildup as the result of cold gas accretion (“cold flows”) feeding star formation in the outer regions while the dense core remains largely unchanged. This scenario naturally explains both the size increase and the fading of the characteristic V‑shaped SED (a strong Balmer break plus UV excess) that originally defined LRDs.

Limitations of the study include (1) the use of a single Sérsic fit, which cannot fully capture the two‑component structure; (2) the neglect of any AGN contribution in the SED modeling, despite evidence that a fraction of LRDs host active nuclei; and (3) the lack of spectroscopic confirmation of the blue envelopes (e.g., emission‑line diagnostics, metallicities, gas kinematics). The authors propose follow‑up observations with JWST/NIRSpec, MIRI, and ALMA to directly measure the gas inflow rates, metallicity gradients, and star‑formation rates in the outskirts, which would provide a decisive test of the cold‑accretion hypothesis.

In summary, this paper presents the first systematic identification of LRD descendants at z < 4, demonstrates that they retain the compact, massive red cores of their progenitors while building up a substantial blue stellar envelope, and shows that their number density evolution is consistent with a simple inside‑out growth scenario driven by cold gas accretion. These findings add a crucial piece to the puzzle of early massive galaxy formation and suggest that the apparent disappearance of LRDs at lower redshift is a selection effect rather than a true disappearance of the objects themselves.