Probing the <i>z</i> ≳ 6 quasars in a universe with IllustrisTNG physics: impact of gas-based black hole seeding models

  • Aklant K Bhowmick
    Department of Physics, University of Florida , Gainesville, FL 32611, USA
  • Laura Blecha
    Department of Physics, University of Florida , Gainesville, FL 32611, USA
  • Yueying Ni
    McWilliams Center for Cosmology , 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
  • Tiziana Di Matteo
    McWilliams Center for Cosmology , 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
  • Paul Torrey
    Department of Physics, University of Florida , Gainesville, FL 32611, USA
  • Luke Zoltan Kelley
    Center for Interdisciplinary Exploration and Research in Astrophysics, Northwestern University , Evanston, IL 60208, USA
  • Mark Vogelsberger
    Department of Physics, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology , Cambridge, MA 02139, USA
  • Rainer Weinberger
    Canadian Institute for Theoretical Astrophysics , 60 St George Street, Toronto, ON M5S 3H8, Canada
  • Lars Hernquist
    Harvard-Smithsonian Center for Astrophysics , 60 Garden Street, Cambridge, MA 02138, USA

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<jats:title>ABSTRACT</jats:title> <jats:p>We explore implications of a range of black hole (BH) seeding prescriptions on the formation of the brightest $z$ ≳ 6 quasars in cosmological hydrodynamic simulations. The underlying galaxy formation model is the same as in the IllustrisTNG simulations. Using constrained initial conditions, we study the growth of BHs in rare overdense regions (forming $\gtrsim 10^{12}\, {\rm M}_{\odot }\,h^{-1}$ haloes by $z$ = 7) using a  (9 Mpc h−1)3 simulated volume. BH growth is maximal within haloes that are compact and have a low tidal field. For these haloes, we consider an array of gas-based seeding prescriptions wherein $M_{\mathrm{seed}}=10^4\!-\!10^6\, {\rm M}_{\odot }\,h^{-1}$ seeds are inserted in haloes above critical thresholds for halo mass and dense, metal-poor gas mass (defined as $\tilde{M}_{\mathrm{h}}$ and $\tilde{M}_{\mathrm{sf,mp}}$, respectively, in units of Mseed). We find that a seed model with $\tilde{M}_{\mathrm{sf,mp}}=5$ and $\tilde{M}_{\mathrm{h}}=3000$ successfully produces a $z$ ∼ 6 quasar with $\sim 10^9\, {\rm M}_{\odot }$ mass and ∼1047 erg s−1 luminosity. BH mergers play a crucial role at $z$ ≳ 9, causing an early boost in BH mass at a time when accretion-driven BH growth is negligible. With more stringent seeding conditions (e.g. $\tilde{M}_{\mathrm{sf,mp}}=1000$), the relative paucity of BH seeds results in a much lower merger rate. In this case, $z$ ≳ 6 quasars can only be formed if we enhance the maximum allowed BH accretion rates (by factors ≳10) compared to the accretion model used in IllustrisTNG. This can be achieved either by allowing for super-Eddington accretion, or by reducing the radiative efficiency. Our results demonstrate that progenitors of $z$ ∼ 6 quasars have distinct BH merger histories for different seeding models, which will be distinguishable with Laser Interferometer Space Antenna observations.</jats:p>

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