Close Encounters of Black Holes, Stars, and Gas in Galactic Nuclei: A Study of the Observational Signatures

Open Access
Author:
Bogdanovic, Tamara
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 04, 2006
Committee Members:
  • Michael Eracleous, Committee Chair
  • Steinn Sigurdsson, Committee Member
  • Mercedes Richards, Committee Member
  • Peter Istvan Meszaros, Committee Member
  • Lee S Finn, Committee Member
Keywords:
  • tidal disruption of a star by a massive black hole
  • massive black hole binaries
  • observational signatures
Abstract:
We have investigated two different physical scenarios in which massive black holes interact with stars or gas. In the first scenario we have modeled the time-variable profiles of the Balmer alpha emission line from the non-axisymmetric disk and debris tail created in the tidal disruption of a solar-type star by a million solar mass black hole. Two tidal disruption event simulations were carried out using a three dimensional relativistic smoothed-particle hydrodynamic (SPH) code, to describe the early evolution of the debris during the first fifty to ninety days. We have calculated the physical conditions and radiative processes in the debris using the photoionization code Cloudy. We model the emission line profiles in the period immediately after the accretion rate onto the black hole became significant. We find that the line profiles at these very early stages of the evolution of the post-disruption debris do not resemble the double peaked profiles expected from a rotating disk since the debris has not yet settled into such a stable structure. As a result of the uneven distribution of the debris and the existence of a "tidal tail" (the stream of returning debris), the line profiles depend sensitively on the orientation of the tail relative to the line of sight. Moreover, the predicted line profiles vary on fairly short time scales (of order hours to days). Given the accretion rate onto the black hole we also model the Balmer alpha light curve from the debris and the evolution of the Balmer alpha line profiles in time. In the second scenario we model the electromagnetic emission signatures of massive black hole binaries (MBHBs) with an associated gas component. The method comprises numerical simulations of relativistic binaries and gas coupled with calculations of the physical properties of the emitting gas. We calculate the accretion powered UV/X-ray and the Balmer alpha light curves and Balmer alpha emission line profiles. The binary plus gas simulations are carried out with a modified version of the parallel tree SPH code Gadget. The heating, cooling, and radiative processes have been evaluated for three different physical scenarios, where the gas is approximated as a black body, a hydrogen-helium gas or a solar metallicity gas. The calculation of the spectrum of the solar-metallicity scenario is carried out with the photoionization code Cloudy. We investigate gravitationally bound, sub-parsec, intermediate phase binaries which are assumed to have gone through dynamical friction phase and scattering due to stars but have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a co-planar binary and co-rotating gas disk around the more massive black hole, suggest that both emission sources, associated with the accreting black holes, lit up after the accretion on the two black holes becomes significant. Periodical outbursts in the X-ray light curve are pronounced and can serve as a fingerprint for this type of binaries. In the case of counter-rotating binaries, the activity of the system at any given time is most likely determined by its orbit and evolutionary phase. The Balmer alpha emission-line profiles also offer strong indications of a binary presence and may be used as a criterion for selecting MBHB candidates for further monitoring from existing archival data. The orbital period and mass ratio of a binary, and in some cases individual black hole masses and parameters of the binary orbit, could be determined from the Balmer alpha light curves and profiles of carefully monitored candidates. At the sub-parsec orbital separations considered here (~0.01 pc) the interactions with the gas are still the dominant mechanism for dissipation of orbital angular momentum. These interactions could significantly expedite the binary merger if they persist at the same level over many orbits. Although systems with the orbital periods studied here are not within the frequency band of the Laser Interferometer Space Antenna (LISA), their discovery is important for understanding of the merger rates of MBHBs and the evolution of such binaries through the last parsec and towards the detectable gravitational wave window.