This project is a comprehensive theoretical study that will provide a picture of how massive black holes (MBHs) in galaxy centers evolved from early cosmic times to the present day. In the first two parts of the three-part study, coupled gravitational N-body and smooth-particle hydrodynamics simulations will be used by the Principal Investigator and a graduate student to study the growth of MBHs in mergers of galaxies. The first part is aimed at understanding when MBHs form bound pairs following galaxy mergers, and when they light up as single or double active galactic nuclei (AGN). Simulations will explore a range of mass ratios and galaxy morphologies. The second part addresses the dynamical, thermodynamic, and accretion evolution of a MBH pair in a merger remnant. The MBH evolution will be simulated at high resolution, self-consistently following the interplay between accretion and dynamics. The third part of the project will use the simulation results to calculate the evolution of MBH populations along cosmic time, predicting how many MBH mergers are expected in the Universe, and how often AGN occur in merging galaxies. The predicted observables will be: (i) frequency of MBHs in galaxies as a function of galaxy mass and cosmic time, (ii) statistics on double AGNs, and on the luminosity functions of AGN at different redshifts and wavelengths, and (iii) potentially detectable gravitational wave event rates from mergers. The proposed research will constrain the dynamics and mass growth of MBHs during mergers and their frequency in galaxies, key information for models of galaxy evolution. The project will contribute to training the next generation of scientists through support of a doctoral student. In an outreach component, undergraduate students will develop critical thinking by designing a museum exhibit dealing with common misconceptions on black holes.

Project Report

One of the goals of current astrophysical research is to understand the connection between galaxies and black holes through their activity, observed in the form of "quasars" or "active galactic nuclei". Quasars are among the brightest objects in the universe (from 5 million to 1 billion times the luminosity of the Sun), and their energy is created by gas falling into a black hole. Scientists seek to determine whether and how the merger of two galaxies can cause the black hole to become "active" and trigger the occurrence of quasars. To this end, we have performed computer simulations of galaxy mergers (taking into account all of their components such as gas, stars, dark matter and black holes) over a period of up to 2 billion years. Analysis of the results showed that the black hole activity takes place mostly during the merger of galaxies but can also be observed, in a lesser extent, when galaxies are in isolation. The results also highlight the influence of the mass and energy of black holes on the properties of galaxies and allow the astrophysical community to compare the models with the observations. A second fundamental question is related to black hole mergers. When two galaxies merge, the fate of the black holes originally present in each galaxy is not straightforward. If the orbital decay is efficient, the two black holes can form a binary when they reach a separation of a few light-years, starting from an initial distance tens of thousands of times larger. How long does it take for the black holes to cross this enormous distance, and how often they are able to find each other at the center of the merger remnant are the two main questions we addressed. We found that, during the late stages of the merger, tidal shocks cause one or both nuclei to be disrupted and leaving their black hole ‘naked’, without any bound gas or stars. When such disruption occurs only in the most massive nucleus, the time-scale for the black holes to bind is much shorter than when the nucleus of the smaller galaxy is disrupted. This suggests that black hole mergers may be more common than previously thought. We also created two museum exhibits to explain different aspects of our research to a large audience. One is a permanent poster exhibit on display at the University of Michigan Museum of Natural History. This poster exhibit explains some of the basics of black holes, including the event horizon, singularity, gravitational attraction, jets, and spins of black holes. The second is a series of computer animations to help educate how black holes form and combat some misconceptions about black holes. In particular, we show how stellar mass black holes form as the deaths of large stars; how supermassive black holes are thought to form through a form of direct collapse; and how black holes are not vacuum cleaners that suck everything into them. These computer animations are designed to make use of digital planetarium equipment but can also be viewed on regular screens. These will be made available to all.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Patricia Knezek
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University of Michigan Ann Arbor
Ann Arbor
United States
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