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Early Formation of Supermassive Black Holes via Dark Star Gravitational Instability

Katherine Freese, George M. Fuller, Sohan Ghodla, Cosmin Ilie, Kyle S. Kehrer, Tanja Rindler-Daller, Evangelos I. Sfakianakis

Abstract

We show that dark stars, which are dark-matter-powered stars in the early universe, can grow by accretion to masses in the range $\mathscr{O}\left ({10}^4\right )-\mathscr{O}\left ({10}^7\right)\,{M_\odot}$ before the general-relativistic Feynman-Chandrasekhar instability causes their dynamical collapse to black holes. These accreting dark star configurations avoid standard stellar nuclear- and weak-interaction evolution that would lead to their demise long before they reached this supermassive size. Remarkably, this mechanism for supermassive black hole (SMBH) genesis is relatively robust to initial dark star mass, formation epoch, accretion rate and its history. The SMBHs produced this way can serve as seeds for even larger SMBHs $({\gtrsim}10^9\,M_\odot)$ that have been discovered at high redshift.

Early Formation of Supermassive Black Holes via Dark Star Gravitational Instability

Abstract

We show that dark stars, which are dark-matter-powered stars in the early universe, can grow by accretion to masses in the range before the general-relativistic Feynman-Chandrasekhar instability causes their dynamical collapse to black holes. These accreting dark star configurations avoid standard stellar nuclear- and weak-interaction evolution that would lead to their demise long before they reached this supermassive size. Remarkably, this mechanism for supermassive black hole (SMBH) genesis is relatively robust to initial dark star mass, formation epoch, accretion rate and its history. The SMBHs produced this way can serve as seeds for even larger SMBHs that have been discovered at high redshift.

Paper Structure

This paper contains 4 equations, 2 figures.

Figures (2)

  • Figure 1: Evolutionary tracks (solid curves) for DSs as calculated with MESA are shown on a plot of the central density $\rho_{\rm c}$vs. the stellar mass $M$. These calculations assume mass accretion at a constant rate of $\dot M = 10^{-2} M_\odot$ yr$^{-1}$. Different colors correspond to different DM particle masses $m_\chi = 1,10,100,10^3,10^4,10^5$ GeV (bottom to top). The dashed curves show the critical density for the onset of GR instability and collapse, Eq. \ref{['eq:critCentralDensity']}. Here, the mean molecular weight $\mu$ is computed self-consistently as a function of $M$ for each star and deviates from the primordial value $\mu=0.59$ (black-dashed curve) only in the case of $m_\chi=100$ TeV (brown-dashed curve). The intersection of the solid and dashed lines for a given color marks the GR instability point in the DS's evolution.
  • Figure 2: The critical mass of a DS at the onset of collapse as a function of the DM particle mass for various mass accretion rates. The critical mass of the DS can be used as an estimate for the mass of the resulting BH. We see that the critical mass is independent of the accretion rate, except for the case of $m_\chi=100$ TeV, as explained in the text.