Little Red Dots on FIRE: Exploring the formation and observational signatures of ultra-compact early galaxies

Niranjan Chandra Roy, Daniel Anglés-Alcázar, Rachel K. Cochrane, Alexander J. Richings, Jonathan Mercedes-Feliz, Christopher C. Hayward, Claude-André Faucher-Giguère, Erini Lambrides, Robert Feldmann, Boon Kiat Oh, Andrew Marszewski, Guochao Sun, Kelcey Davis, Jed McKinney, Caitlin M. Casey, Tanio Díaz-Santos, Madisyn Brooks, Grace Farrell

First listed 2026-06-23 | Last updated 2026-06-22

Abstract

Little Red Dots (LRDs) are compact sources with broad Balmer lines, Balmer breaks, anomalous UV emission, rising red continuum, and uncertain origin. We use FIRE cosmological simulations, 3D dust radiative transfer, and synthetic emission-line data cubes to test whether ultra-compact early galaxies can reproduce LRD-like observables without invoking AGN. In progenitors of present-day group halos ($M_{\rm halo} > 10^{13.5} M_{\odot}$), we identify transient phases at $z \approx 4-8$ lasting $\sim 150-400$ Myr in which strong dissipative inflows build massive ($M_{\star} \sim 10^{8.5}-10^{10.5} M_{\odot}$), UV-bright ($-23 \lesssim M_{\rm UV} \lesssim -20$), ultra-compact ($R_{\rm eff} < 300$ pc) stellar cores with extreme circular velocity ($V_{\rm circ} > 500$ km s$^{-1}$) and consistent with several LRD properties: strong Balmer breaks ($F_ν(4200{\rm Å})/F_ν(3500{\rm Å}) \sim 2$); blue UV beta slopes ($β_{\rm UV} \approx -1.25$); dust masses; ALMA non-detections; and Balmer-line widths up to $\sim 1500$ km s$^{-1}$ broadened by galaxy-scale dynamics. However, stellar emission and host-galaxy kinematics alone do not reproduce the red rest-optical continuum, more extreme Balmer breaks ($\gtrsim 2.5$) and line widths ($\gtrsim 2000$ km s$^{-1}$), or the broad-Balmer/narrow-forbidden-line signature of broad-line AGN. The same ultra-compact conditions efficiently fuel central BHs, suggesting a hybrid stellar+AGN scenario in which compact stars explain the UV continuum, Balmer break, and intermediate line widths while AGN supply the red optical continuum and more extreme line properties. With halo masses $M_{\rm halo} \sim 10^{11-12.5} M_\odot$ and comoving abundance $\sim 2 \times 10^{-5} {\rm cMpc}^{-3}$ (for $\sim 20\%$ duty-cycle at $z \approx 4-8$), ultra-compact galaxies can contribute to the massive, bright LRD population.

Short digest

Using FIRE zoom-in simulations with 3D dust radiative transfer and synthetic line cubes, this paper asks how far a stellar-only explanation for little red dots can go. In progenitors of present-day group halos, the authors find transient z≈4–8 compaction phases lasting roughly 150–400 Myr that build ultra-compact, UV-bright stellar cores with R_eff<300 pc, V_circ>500 km s^-1, Balmer-break strengths around 2, blue UV slopes, ALMA-faint dust emission, and Balmer-line widths up to about 1500 km s^-1 from galaxy-scale dynamics. The key result is that these compact galaxies can reproduce a substantial subset of LRD observables without AGN, especially the UV continuum, moderate Balmer breaks, compactness, and intermediate broad-line widths. But the simulations do not recover the reddest rest-optical continua, the most extreme Balmer breaks or >2000 km s^-1 lines, or the broad-Balmer plus narrow-forbidden-line combination, so the paper argues for a hybrid compact-stars plus AGN interpretation for at least the bright massive LRD population.

Key figures to inspect

• Figure 1. Use this as the physical setup and timescale figure. It shows how galaxy B2 enters a roughly 400 Myr ultra-compact phase with R_eff below 300 pc while the central 100 pc reaches extreme stellar surface density and V_circ≈800 km s^-1, directly motivating the paper’s claim that dissipative inflows can assemble LRD-like stellar cores.
• Figure 2. This is the clearest population-level selection figure. It demonstrates that only the earlier-growing, more massive halo tracks enter the LRD-like region in the V_circ–R_eff plane at z≈4–8, linking the proposed stellar-only channel to progenitors of present-day group-scale environments rather than to the full simulated sample.
• Figure 5. This figure is central for the observables case. It shows that during compact phases the synthetic SEDs produce Balmer breaks near the observed LRD range and blue UV slopes, while also making clear the role of dust and viewing angle in shaping the emergent continuum.
• Figure 7. Choose this for the quantitative line-width test. It tracks Hβ and [O III] width evolution versus redshift and projection, showing that host-galaxy dynamics can push widths into the LRD-like intermediate regime but also illustrating the limit of the stellar-only scenario relative to the most extreme broad-line sources.
• Figure 9. Include this as an important observational cross-check. It shows that the simulated compact phase remains undetectable in ALMA dust continuum in one-hour integrations across bands, supporting the paper’s claim that ALMA non-detections are compatible with ultra-compact stellar systems of this kind.