Constraints on dynamically-formed massive black holes in Little Red Dots from X-ray non-detections

M. Liempi, D. R. G. Schleicher, M. A. Latif, R. Schneider, F. Flammini Dotti, A. Escala, M. C. Vergara

First listed 2026-02-24 | Last updated 2026-02-25

Abstract

The existence of massive, compact galaxies (Little Red Dots, LRDs) at $z \sim 2$ challenges early structure formation models, suggesting rapid stellar and black hole (BH) assembly. While LRDs are efficient environments for BH growth, many show no X-ray evidence of strong AGN emission. We utilize a subsample of X-ray non-detected LRDs to test the compatibility of collision-based BH formation scenarios and constrain physical parameters like metallicity and column density. Our results indicate LRDs are ideal birthplaces for massive BHs, particularly given a mass-radius relation $R_{gal} \propto M_{gal}^{0.6}$. Collision-based models suggest seed masses larger than those in the local Universe, consistent with high-redshift BH mass-radius relations. We modeled BH seed formation and X-ray emission (0.3-7 keV) against observed upper limits. We find that mass-radius exponents $> 0.55$ favor the collision-based scenario; however, consistency with stacked X-ray analysis requires specific accretion and obscuration parameters. Constant or increasing SFR scenarios with high Eddington ratios are feasible but necessitate larger column densities or higher metal enrichment. Alternatively, moderate sub-Eddington accretion reconciles massive seeds with observed masses and X-ray weakness. We conclude that even if LRDs began as starbursts, they should eventually evolve into AGNs.

Short digest

Using a stack of 55 X-ray–non-detected Little Red Dots from JADES and NGDEEP, the authors model collision-driven black-hole seeding and 0.3–7 keV emission against the Chandra upper limits. They find that compact mass–radius scalings with exponents >0.55 (e.g., R_gal ∝ M_gal^0.6) naturally yield massive seeds and are compatible with the stacked non-detections under specific accretion/obscuration combinations. High Eddington ratios remain viable only with large columns and/or enhanced metallicity in the shielding gas, while moderate sub-Eddington growth also reconciles seed and final masses with the observed X-ray weakness. The upshot is that LRDs are prime birthplaces for massive BHs and, even if initially starbursts, should evolve into AGN.

Key figures to inspect

• Sample definition and redshift–mass panel: compare the 55 stacked sources to the 341-object parent LRD catalog; check how M* was inferred from M_UV and how the JADES/NGDEEP subsample spans z and luminosity.
• Galaxy mass–radius scaling constraints: plot of the assumed R_gal–M_gal relation; inspect which exponents (>0.55) keep collision-driven seeds consistent with X-ray limits and how R ∝ M^0.6 changes predicted seed masses.
• Predicted L_X versus stacked upper limits: curves for different accretion rates and duty cycles against the soft/hard-band non-detections; identify regions excluded by the stack.
• Allowed parameter space in (λ_Edd, N_H, Z): map showing that high Eddington ratios require larger columns and/or higher metallicity, while sub-Eddington tracks satisfy limits with milder obscuration.
• Seed-to-final BH mass evolution: trajectories demonstrating how collision-born seeds grow under constant or rising SFR scenarios and remain X-ray weak while reaching observed masses.