Open Access
Micic, Miroslav
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
August 01, 2007
Committee Members:
  • Steinn Sigurdsson, Committee Chair
  • Kelly Holley Bockelmann, Committee Member
  • Donald P Schneider, Committee Member
  • Robin Bruce Ciardullo, Committee Member
  • Jane Camilla Charlton, Committee Member
  • Ben Owen, Committee Member
  • SMBH
  • gravitational waves
  • galaxy formation
  • merger rates
  • ULX
Understanding how seed black holes grow into intermediate and supermassive black holes (IMBHs and SMBHs, respectively) has important implications for the dutycycle of active galactic nuclei (AGN), galaxy evolution, and gravitational wave astronomy. Primordial stars are likely to be very massive &#8805;30M&#8857;, form in isolation, and will likely leave black holes as remnants in the centers of their host dark matter halos. We expect primordial stars to form in halos in the mass range 10^6 &#8722;10^10M&#8857;. Some of these early black holes, formed at redshifts z>10, could be the seed black hole for a significant fraction of the supermassive black holes found in galaxies in the local universe. If the black hole descendants of the primordial stars exist, their mergers with nearby supermassive black holes may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter halos in LambdaCDM universe. We seed high-redshift dark matter halos with early black holes, and explore the merger history of the host halos and the implications of black hole’s kick velocities arising from their coalescence. The central concentration of low mass early black holes in present day galaxies is reduced if they experience even moderate kicks of tens of kms&#8722;1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low mass black holes that were ejected, compared to those still embedded in their parent halos. Therefore, merger rates with central supermassive black holes in the largest halos may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N–body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal free stars with supermassive black holes in present day galaxies. Most studies of the cosmological growth and merger history of black holes have used semianalytic models and have concentrated on SMBH growth in luminous galaxies. We have developed a ”hybrid method” that combines high resolution cosmological Nbody simulations for the halos’ merger history, with semi-analytical recipes for BH pair dynamics and BH gas accretion. We track the assembly of black holes over a large range of final masses – from seed black holes to SMBHs – over widely varying dynamical histories. We used the dynamics of dark matter halos to track the evolution of seed black holes in three different gas accretion scenarios. We have found that growth of a Sagittarius A* - size SMBH reaches its maximum mass MSMBH=10^6 M&#8857; at z&#8764;6 through early gaseous accretion episodes, after which it stays at near constant mass. At the same redshift, the duty-cycle of the host AGN ends, hence redshift z=6 marks the transition from an AGN to a starburst galaxy which eventually becomes the Milky Way. By tracking black hole growth as a function of time and mass, we estimate that the IMBH merger rate reaches a maximum of Rmax=55 yr&#8722;1 at z=11. From IMBH merger rates we calculate the number density of ultra-luminous X-ray sources (ULX) to be NULX=7 per Milky Way type galaxy per redshift in redshift range 2<z<6. We model gravitational wave signatures for various SMBH growth scenarios and find that the future gravitational observatory LISA, will be able to detect mergers of massive black holes up to redshift z=5. We also analyse how the merger rate changes if various suppression mechanisms are implemented in merger rates and find that low seed formation rate will substantially decrease the overall merger rate to &#8764; 10 per year; low efficiency in dynamical friction will influence low redshift merger rates; and gravitational recoil from the mergers will eject high redshift black holes from their host halos. All three suppression mechanisms combined reduce merger rate to &#8764; 1 per year.