LRDs: Compact Cores in High-Redshift Galaxies

• LRDs are extremely compact (Re ~100 pc), red high-redshift sources defined by distinctive V-shaped SEDs with steep UV and red optical slopes.
• They exhibit high stellar mass surface densities (~10^11–10^12 M⊙ kpc⁻²) and often show broad Balmer emission lines, indicating active SMBH accretion.
• Evolutionary studies suggest LRDs form dense galaxy cores that gradually build blue, star-forming outskirts through inside-out growth and cold gas accretion.

Little Red Dots (LRDs) are a population of extremely compact, red high-redshift ($z \gtrsim 4$) sources identified primarily in JWST surveys by their distinctive "V-shaped" rest-frame spectral energy distributions (SEDs), which display a flat or blue ultraviolet continuum transitioning sharply to a much redder optical continuum. LRDs are characterized by high stellar mass surface densities, broad Balmer emission lines (signaling AGN activity in many cases), and compact morphologies (typical $R_{\rm eff} \sim 100$ pc) that place them far below the canonical size–mass relation for galaxies at similar redshifts. Extensive spectroscopic, photometric, and morphological analysis indicates that LRDs represent the dense inner cores of forming massive galaxies, often powered or influenced by newly grown or rapidly accreting supermassive black holes (SMBHs), and that their compact red appearance is closely linked to both intrinsic evolutionary states and observational selection effects.

1. Core Properties and Identification Criteria

LRDs are empirically defined through photometric color/SED selection techniques that leverage their unique combination of extreme compactness and SED slopes:

• The UV-optical SED shows a steeply negative UV slope ($\beta_{\rm UV} < -0.37$) and a positive optical slope ($\beta_{\rm opt} > 0$), producing the canonical V-shape (Hainline et al., 30 Sep 2024).
• Key color criteria (e.g., F444W–F277W $>$ 1.5 mag) and compactness metrics (inner-to-total flux ratios in NIRCam images) ensure the exclusion of extended or blue-dominated sources (Feeney et al., 20 Sep 2024, Billand et al., 5 Jul 2025).
• Morphologically, they have half-light radii as small as $R_e \lesssim 100$ pc, corresponding to central stellar mass surface densities $\Sigma_* \sim 10^{11}$–$10^{12}~M_\odot~\mathrm{kpc}^{-2}$.
• Spectra often reveal broad Balmer emission lines with FWHM $\gtrsim 1000~\mathrm{km~s^{-1}}$, cementing an AGN association in many cases, but a fraction show only narrow lines and are interpreted as compact, dusty starbursts or low-mass, early AGN (Zhang et al., 4 Jun 2025).
• Stellar mass estimates for the red compact core are typically $M_* \sim 10^{10}~M_\odot$, and effective radii $R_e$ of candidate "descendants" remain at $250$–$600$ pc for $z=5$–$3$ (Billand et al., 5 Jul 2025).

2. Physical Evolution and Morphological Growth

Analysis of LRDs across redshift reveals systematic trends consistent with inside-out galaxy growth:

• At $z \gtrsim 5$, LRDs are almost unresolved, with nearly all stellar mass contained in the red, dense core.
• At lower redshifts ($z \sim 3$), a significant fraction of stellar mass ($f_{\rm outskirt} \sim 0.3$–$0.5$) is in blue, star-forming outskirts, and the effective radius grows up to $\sim 600$ pc. This radial growth co-occurs with a fading of the V-shaped SED as the larger, bluer envelope dominates the integrated light (Billand et al., 5 Jul 2025).
• The core remains dense and red, retaining high $\Sigma_*$, while new, young stars are assembled in the outskirts by cold gas accretion (direct infall of pristine gas) and possibly minor mergers (i.e., dry mergers that build stellar mass without significantly raising star formation rates).
• The observed number density of these post-LRD systems at $z=3 \pm 0.5$ ($\sim 10^{-4.15}$ Mpc$^{-3}$) matches that of LRDs at $5

3. SED Modeling and Stellar/AGN Decomposition

SED fitting reveals the necessity of multi-component modeling to interpret the energy budgets of LRDs and their progeny:

• SED models that attribute all red continuum to stars require extremely high stellar mass densities and sometimes unphysical baryon conversion efficiencies, challenging pure galaxy scenarios (Leung et al., 18 Nov 2024).
• Pure AGN models can explain the V-shaped SEDs, but yield supermassive black hole masses that are inconsistent with virial line estimates unless non-Eddington accretion rates or partial stellar contributions are allowed.
• Hybrid models, where the red continuum is partially AGN-driven and the UV is stellar, alleviate tension with both extreme mass densities and overmassive BHs, and better account for observed emission-line and continuum properties. The red optical bump is frequently attributed to either blackbody emission from a photosphere/envelope or from the outer/dusty regions of an accretion disk (Zhang et al., 19 May 2025, Kido et al., 11 May 2025).

4. Observational Biases and the Nature of Compactness

Much of the compact, red appearance of high-$z$ LRDs is driven by observational effects:

• Cosmological surface brightness dimming scales as $(1+z)^{-4}$, so extended emission from LRD host galaxies falls rapidly below detectability at high $z$. Analytic models show that $>70\%$ of the extended light of a galaxy like "The Saguaro" at $z = 2$ would be lost if moved to $z=7$ (Rinaldi et al., 23 Jul 2025).
• NIRCam’s long-wavelength filter PSFs further blend diffuse outskirts into the nuclear component, emphasizing the core.
• Stacking analyses of LRDs in the rest-UV show faint, diffuse halos surrounding the nucleus, indicating that at least some LRDs are in fact the visible nuclei of more extended galaxies whose outskirts are hidden below the noise threshold.
• These effects explain the transition from “naked” compact LRDs at high $z$ to galaxies like WISEA J123635.56+621424.2 (Saguaro) at lower $z$, which has a clear spiral host galaxy but would appear as an LRD if redshifted to $z=7$ (Rinaldi et al., 23 Jul 2025).

5. Descendants and the Decline in LRD Number Density

The apparent two-order-of-magnitude decline in LRD abundance from $z=6$ to $z=3$ is explained as a selection effect resulting from galaxy growth:

• As blue star-forming outskirts are acquired and dominate the integrated light, the distinctive V-shaped SED disappears and the compactness criterion is no longer satisfied.
• However, the "post-LRD" descendants retain dense red cores, similar central stellar densities, and total masses consistent with high-$z$ LRDs, only now wrapped in a larger, younger envelope. Thus, the true abundance of LRD progeny is likely underestimated if only compact, V-shaped SED objects are counted (Billand et al., 5 Jul 2025).
• This scenario implies that LRD-like nuclei may be common in massive galaxies at later cosmic times, but increasingly "hidden" by growing hosts.

6. Implications for Galaxy and SMBH Evolution

LRDs provide a direct empirical probe of the early assembly of massive galaxies and their central black holes:

• Their properties (extremely high central densities, early onset of massive, compact stellar cores, frequent AGN signatures) support scenarios in which inside-out growth builds extended galaxies around dense progenitor nuclei.
• The presence of broad-line AGN in many LRDs suggests that SMBH growth starts very early in the most massive and compact environments, frequently reaching "overmassive" states compared to the $M_{\rm BH}$–$M_*$ relation observed locally (Lin et al., 11 Dec 2024).
• The lack of strong X-ray/radio emission and limited variability in many LRDs may reflect deeply embedded accretion in high-column-density environments ("black hole envelopes" or cocooned AGN) (Kido et al., 11 May 2025, Rusakov et al., 20 Mar 2025).
• Stacked and morphological analyses, combined with careful SED modeling, indicate that AGN feedback and cold gas accretion jointly set the observed evolutionary sequence: early compaction, onset of AGN, formation of blue outskirts, and eventual emergence as extended, massive, and less compact galaxies at lower $z$.

7. Future Directions

Outstanding challenges and prospects include:

• High-resolution, multi-wavelength follow-up (e.g., JWST/NIRSpec IFU, ALMA, deep imaging) to spatially resolve star formation, metallicity, and kinematics in both cores and outskirts.
• Improved SED decomposition methods to constrain the balance of AGN vs. star formation in LRDs and their descendants.
• Statistical studies of larger LRD samples at $z > 7$ to directly observe the onset of blue envelope growth and better map the redshift evolution of compactness and stellar density.
• Refined theoretical models of core formation, AGN feedback, and gas accretion to explain the extreme densities and dual AGN–starburst nature of these systems.

In sum, LRDs and their evolutionary descendants provide a laboratory for dissecting the earliest phases of massive galaxy and SMBH growth, offering direct constraints on core formation, assembly by cold accretion, feedback-regulated growth, and the interplay of observational selection effects with intrinsic galactic evolution (Billand et al., 5 Jul 2025, Rinaldi et al., 23 Jul 2025).