The Evolving Population of Black Holes
Four galaxies simulated by the Illustris program, which also tracks the evolution of the supermassive black holes at their centers.
Quasars are among the most luminous sources in the universe, shining with a total luminosity of hundreds or thousands of Milky Ways. Their large radiative emission enable us to observe them out to great distances and thus to investigate their evolution over more than ninety per cent of cosmic time. The mechanism most likely to power quasars is accretion onto the supermassive black hole at each galaxy's nucleus.
Although there is a difference of a factor of about one billion in the physical size scales between a black hole's accreting environment and its host galaxy size, the two dimensions are closely correlated in the many observed objects, suggesting that there is some kind of feedback between the growth of the black hole and that of its host galaxy. Understanding what those feedback mechanisms are, and how they affect the growth of the galaxy (in particular its star formation), are of paramount importance for understanding galaxy formation and evolution. Both processes are thought to have peaked in activity when the universe was only a few billion years old, but neither is particularly well understood. Astronomers have a generally good understanding of the range of properties and activity of quasars and their black holes in the local universe, but know much less about quasars dating from the first billion years of the universe.
CfA astronomers Shy Genel, Paul Torrey, Dylan Nelson, and Lars Hernquist and their colleagues have developed a computer simulation that attempts to address these mysteries. "Illustris" is the first simulation capable of modeling galaxy evolution at scales down to relatively small sizes – only a few thousand light-years – a capability that in particular allows the code to track the development and growth of the supermassive nuclear black holes. The code assumes basic cosmological parameters, including dark matter, and by the time the simulation ends in the current epoch it has produced 32,542 black holes, 3,965 of them more massive than ten million suns.
When compared with recent observational constraints, the Illustris simulation results are in very good agreement, providing confidence in the accuracy of the new conclusions. These include finding that black holes and galaxies evolve in mass together, at least in the case of massive galaxies, that there is a so-far unobserved population of faint, active black holes in the early universe, and that the efficiency at which black hole accretion mechanisms radiate is only about ten percent of the theoretical maximum. The new results show the unique role that large-scale simulations play in filling in the missing gaps in our understanding of the early evolution of galaxies and their black holes.
"The Illustris Simulation: The Evolving Population of Black Holes Across Cosmic Time," Debora Sijacki, Mark Vogelsberger, Shy Genel, Volker Springel, Paul Torrey, Gregory F. Snyder, Dylan Nelson and Lars Hernquist, MNRAS, 452, 575, 2015.