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Supermassive black holes swallow stellar objects at high rates: from Little Red Dots to Black Hole Stars

Konstantinos Kritos, Joseph Silk

TL;DR

The paper investigates rapid SMBH growth at high redshift by quantifying tidal-disruption and compact-object capture rates in nuclear star clusters surrounding SMBHs. It combines loss-cone theory, Monte Carlo NSC modeling, and updated stellar evolution to predict TDE and EMRI rates, along with the associated electromagnetic transients and gravitational-wave signals, across redshifts up to $z\sim6$. The main result is a high intrinsic MS TDE rate at $4\le z\le6$ of $\sim5\times10^{3}\ \mathrm{Gpc^{-3}\,yr^{-1}}$, capable of producing extreme nuclear transients, while WD/NS/BH captures generate a detectable stochastic GW background and numerous GW sources for LISA/LGWA, offering direct probes of early SMBH assembly. The findings underscore that stellar feeding, though insufficient alone for SMBH mass growth, leaves observable imprints through ENTs and GW signals and supports a BH–star cocoon picture for Little Red Dots."

Abstract

Supermassive black hole growth plausibly occurs via runaway astrophysical black hole mergers in nuclear star clusters that form intermediate mass black hole seeds at high redshifts. Such a model of Little Red Dots yields an order-of-magnitude higher rate of tidal disruption events than that of black hole captures. Our prediction, normalised to our proposed resolution of SMBH seeding, yields detectable TDE rates at high redshift. The resulting dense gas cocoons generate the nuclei of LRDs, each incorporating a central massive black-hole-star, with comparable masses in gas, stars, and massive black hole within a scale of around a parsec as inferred from the various spectral signatures.

Supermassive black holes swallow stellar objects at high rates: from Little Red Dots to Black Hole Stars

TL;DR

The paper investigates rapid SMBH growth at high redshift by quantifying tidal-disruption and compact-object capture rates in nuclear star clusters surrounding SMBHs. It combines loss-cone theory, Monte Carlo NSC modeling, and updated stellar evolution to predict TDE and EMRI rates, along with the associated electromagnetic transients and gravitational-wave signals, across redshifts up to . The main result is a high intrinsic MS TDE rate at of , capable of producing extreme nuclear transients, while WD/NS/BH captures generate a detectable stochastic GW background and numerous GW sources for LISA/LGWA, offering direct probes of early SMBH assembly. The findings underscore that stellar feeding, though insufficient alone for SMBH mass growth, leaves observable imprints through ENTs and GW signals and supports a BH–star cocoon picture for Little Red Dots."

Abstract

Supermassive black hole growth plausibly occurs via runaway astrophysical black hole mergers in nuclear star clusters that form intermediate mass black hole seeds at high redshifts. Such a model of Little Red Dots yields an order-of-magnitude higher rate of tidal disruption events than that of black hole captures. Our prediction, normalised to our proposed resolution of SMBH seeding, yields detectable TDE rates at high redshift. The resulting dense gas cocoons generate the nuclei of LRDs, each incorporating a central massive black-hole-star, with comparable masses in gas, stars, and massive black hole within a scale of around a parsec as inferred from the various spectral signatures.
Paper Structure (10 sections, 10 equations, 3 figures, 4 tables)

This paper contains 10 sections, 10 equations, 3 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Stellar feeding onto central hole
  3. Initial conditions
  4. Mass accretion rate
  5. Flaring events
  6. Integration over cosmological volume
  7. Gravitational-wave background
  8. Formation of the black-hole star
  9. Conclusions
  10. Extended tables of loss-cone rates

Figures (3)

  • Figure 1: Temporal mass accretion rates from captures onto $10^5\,M_\odot$ (dotted), $10^6\,M_\odot$ (solid), and $10^7\,M_\odot$ (dashed) SMBHs, decomposed by stellar type via loss-cone contributions. Abbreviations: MS (main sequence); WD (white dwarf); NS (neutron star); BH (black hole). Points with error bars indicate mean values and standard deviations from ten realizations. The inset displays the duty cycle of MS (red) and giant (blue) stars.
  • Figure 2: Peak luminosity versus characteristic rest-frame time-scale of astronomical transients, with peak absolute magnitudes normalized to a solar V-band magnitude of 4.83; ${\rm M}_{\rm pk}=2.5\log_{10}(L_{\rm pk}/L_\odot)-4.83$. Tidal disruption flares of MS stars by SMBHs of $10^{5}\,M_\odot$ (crosses), $10^{6}\,M_\odot$ (circles), and $10^{7}\,M_\odot$ (diamonds) are shown as colored symbols, with the color bar indicating stellar mass. Five distinct sets correspond to five evolutionary times (bold labels). The six red symbols denote observed flares with $L_{\rm pk}>10^{45}\,\rm erg\,s^{-1}$. Gray dotted horizontal lines mark the Eddington luminosities for the labeled SMBH masses. Abbreviations: ENTs (extreme nuclear transients); SLSNe (superluminous supernovae); ANTs (ambiguous nuclear transients); TDEs (tidal disruption events); ccSNe (core-collapse supernovae); IaSNe (Type Ia supernovae) following the Phillips relationship.
  • Figure 3: Total dimensionless gravitational-wave (GW) energy density from BH (black), NS (yellow), and WD (cyan) captures within $z<6$ as a function of the observed GW frequency. The comoving SMBH number density is assumed constant at $10^{-2}\,\rm Mpc^{-3}$. Gray curves indicate the PLS of LISA and LGWA with an $\rm SNR=10$ and $T_{\rm obs}=4\,\rm yr$. Also shown are the GW backgrounds from mergers of massive BHs under the light- and heavy-seed scenarios Caliskan:2025esi. The inset shows the characteristic strain for a few $z\sim5$ captures.