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

TL;DR

This study investigates how massive black holes could form in early compact galaxies known as Little Red Dots, using X-ray non-detections to constrain formation models and physical parameters.

Contribution

It provides new constraints on collision-based black hole seed formation scenarios and the physical conditions in high-redshift compact galaxies.

Findings

LRDs are ideal sites for massive BH formation based on their mass-radius relation.

Collision models suggest seed masses larger than local Universe counterparts.

X-ray non-detections constrain accretion and obscuration parameters for BH growth.

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.