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

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.

Accepted for publication 23/02/2026 (A&A)

References in corpus (55)

• On the Absorption of X-rays in the Interstellar Medium
• Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
• The M-sigma and M-L Relations in Galactic Bulges and Determinations of their Intrinsic Scatter
• The size distribution of galaxies in the Sloan Digital Sky Survey
• Nuclear Activity in Nearby Galaxies
• Relations Between Central Black Hole Mass and Total Galaxy Stellar Mass in the Local Universe
• BAT AGN Spectroscopic Survey - V. X-ray properties of the Swift/BAT 70-month AGN catalog
• The XMM-Newton view of PG quasars I. X-ray continuum and absorption
• Obscured Active Galactic Nuclei
• Nuclear Star Clusters
• Gas metallicity diagnostics in star-forming galaxies
• JADES. The diverse population of infant Black Holes at 4<z<11: merging, tiny, poor, but mighty
• Universal bolometric corrections for AGN over 7 luminosity decades
• The Evolution of the Galaxy Stellar Mass Function at z= 4-8: A Steepening Low-mass-end Slope with Increasing Redshift
• Galaxy And Mass Assembly (GAMA): mass-size relations of z$<$0.1 galaxies subdivided by Sérsic index, colour and morphology
• The mass evolution of the first galaxies: stellar mass functions and star formation rates at $4<z<7$ in the CANDELS GOODS-South field
• X-CIGALE: fitting AGN/galaxy SEDs from X-ray to infrared
• The origins of massive black holes
• Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass to Light Ratios from z 7 to z 4
• Hyper-Eddington accretion flows onto massive black holes
• Spin, Accretion and the Cosmological Growth of Supermassive Black Holes
• Masses and Scaling Relations for Nuclear Star Clusters, and their Coexistence with Central Black Holes
• Are Dusty Galaxies Blue? Insights on UV Attenuation from Dust-Selected Galaxies
• Galaxy Stellar Mass Functions from z~10 to z~6 using the Deepest Spitzer/IRAC Data: No Significant Evolution in the Stellar-to-Halo Mass Ratio of Galaxies in the First Gyr of Cosmic Time
• Unifying X-ray Scaling Relations from Galaxies to Clusters
• How Do Star-Forming Galaxies at Z>3 Assemble Their Masses?
• The impact of chemistry on the structure of high-z galaxies
• RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z\sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec
• JWST UNCOVER: Extremely red and compact object at$z_{\mathrm{phot}}\simeq 7.6$ triply imaged by Abell 2744
• Stochastic star formation in early galaxies: JWST implications
• Super-Eddington Accretion and Feedback from the First Massive Seed Black Holes
• EPOCHS VI: The Size and Shape Evolution of Galaxies since z ~ 8 with JWST Observations
• Formation of massive seed black holes via collisions and accretion
• X-ray spectral analyses of AGNs from the 7Ms Chandra Deep Field-South survey: the distribution, variability, and evolution of AGN's obscuration
• The ALMA REBELS Survey: Specific Star-Formation Rates in the Reionization Era
• A dormant, overmassive black hole in the early Universe
• Deciphering the JWST spectrum of a 'little red dot' at $z\sim 4.53$: An obscured AGN and its star-forming host
• Long-Term X-ray Variability of Typical Active Galactic Nuclei in the Distant Universe
• Making a supermassive star by stellar bombardment
• JWST FRESCO: a comprehensive census of H$\beta$+[OIII] emitters at 6.8<z<9.0 in the GOODS fields
• Formation of supermassive stars in the first star clusters
• Simulations predict intermediate-mass black hole formation in globular clusters
• HYPerluminous quasars at the Epoch of ReionizatION (HYPERION). A new regime for the X-ray nuclear properties of the first quasars
• Origin of supermassive black holes in massive metal-poor protoclusters
• Galaxy Size and Mass Build-up in the First 2 Gyrs of Cosmic History from Multi-Wavelength JWST NIRCam Imaging
• Global instability by runaway collisions in nuclear stellar clusters: Numerical tests of a route for massive black hole formation
• Super-Eddington growth of black holes in the early Universe: effects of disk radiation spectra
• The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
• Observational Support for Massive Black Hole Formation Driven by Runaway Stellar Collisions in Galactic Nuclei
• A Highly Magnified Gravitationally Lensed Red QSO at z = 2.5 with a Significant Flux Ratio Anomaly
• Little Red Dots Are Nurseries of Massive Black Holes
• Rapid formation of a very massive star >50000 $M_{\odot }$ and subsequently an IMBH from runaway collisions. Direct N-body and Monte Carlo simulations of dense star clusters
• Efficiency of black hole formation via collisions in stellar systems: An analysis of data from simulations and observations
• Efficient black hole seed formation in low metallicity and dense stellar clusters with implications for JWST sources
• Formation of supermassive stars in the first stellar clusters: Dependence on the gas temperature