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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

TL;DR

This paper investigates the evolutionary fate of Little Red Dots (LRDs) by searching for post-LRD descendants at $z<4$ in the CEERS field, assuming a single pathway where a blue star-forming outskirts grows around a red core. Using color–magnitude criteria to identify red cores with blue outskirts, single-Sérsic morphology in the F444W band, and stellar-only SED fitting for inner and outskirts components, the authors assemble a final sample of 55 galaxies with median redshift $z_{ ext{med}}=3.6$ and $M_ ast \

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.

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

TL;DR

This paper investigates the evolutionary fate of Little Red Dots (LRDs) by searching for post-LRD descendants at in the CEERS field, assuming a single pathway where a blue star-forming outskirts grows around a red core. Using color–magnitude criteria to identify red cores with blue outskirts, single-Sérsic morphology in the F444W band, and stellar-only SED fitting for inner and outskirts components, the authors assemble a final sample of 55 galaxies with median redshift and $M_ ast \

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 to . 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 , 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 , central densities ( ), compact sizes, and red rest-frame colors, but with an added extended component. Their number density at ( ) matches that of LRDs at , supporting a possible evolutionary link. We observe a redshift-dependent increase in outskirts mass fraction and galaxy size-from pc at to pc at -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.

Paper Structure

This paper contains 14 sections, 5 equations, 15 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data and Sample Selection
  3. CEERS data
  4. CEERS catalog and pre-selection
  5. Sample selection
  6. Final sample
  7. Inferring physical properties
  8. Spectral Energy Distribution fitting
  9. Morphological analysis
  10. Relation with LRDs
  11. The building of an outskirt
  12. Discussion
  13. Conclusion
  14. Outskirt detectability and surface brightness dimming

Figures (15)

  • Figure 1: Color magnitude diagram illustrating the parent sample : in purple we show the sources that respect the color-magnitude criterion, and black edges denotes the parent sample, respecting $z>2.5$. In red, we show the color-magnitude selection for the red color of LRDs F277W - F444W > 1.5 used in akins_cosmos-web_2024, having a $z_{median}\approx 6.5$. Red and purple contours represent all CEERS sources at 25%, 50% and 80% levels shown for F150W-F356W.
  • Figure 2: Example illustrating the definition of the outskirts and inner regions. Left panel: stacked image combining all JWST bands, defined as STACK = F115W + F150W + F200W + F277W + F356W + F410M + F444W, using PSF-matched data. The dotted line shows the segmentation map, while the solid line traces the ellipse with a semi-major axis of $0.16^"= 2\text{HWHM}$. Right panel: color composite image using F150W (blue), F277W (green), and F444W (red). The white circle indicates the PSF FWHM beam at F444W. As expected, the galaxy exhibits a centrally concentrated red emission and a bluer, more extended periphery.
  • Figure 3: Redshift distribution of the final sample in purple. 6 LRDs from our sample were also in kocevski_rise_2024 LRDs sample, shown in red.
  • Figure 4: Error in % of the half-light radii $R_e$ and Sérsic index $n$ as a function of the inferred half-light radii for mock sources injected into dark areas of the CEERS field of view with mag = 26. The error is defined as $\text{Err}_\text{tot} =\sqrt{\text{Err}_{R_e}^2 + \text{Err}_n^2}$. The dotted line denotes the HWHM in F444W, whereas the dash-dotted line refers to the resolution limit adopted in this study.
  • Figure 5: Mass distribution for several samples of LRDs from previous works, assuming only a stellar model, from rinaldi_not_2024perez-gonzalez_what_2024akins_cosmos-web_2024leung_exploring_2024labbe_population_2023. This sample is visible in purple for both the inner part and the total mass.
  • ...and 10 more figures