Massive binary black holes in the cosmic landscape

Monica Colpi

Binary black holes occupy a special place in our quest for understanding the formation and evolution of galaxies along cosmic history. If massive black holes grow at the center of pre-galactic structures that experience a sequence of merger episodes, then "binary" black holes inescapably form. If the binary black holes end coalescing inside the remnant galaxy, then they become powerful sources of gravitational waves, visible out to very large redshifts when the universe was only a few hundred million years old. If detected in the new window provided by the upcoming gravitational wave experiments such as the Laser Interferometer Space Antenna (LISA), coalescing binary black holes will provide a deep view into the process of hierarchical clustering which is at the heart of the current paradigm of galaxy formation. LISA will determine with astonishing accuracy the mass and spin of coalescing black hole binaries providing clues on how black holes grow and co-evolve with their hosts. After reviewing current studies on the cosmic assembly of galaxies with black holes and recent observations of albeit rare, binary black hole candidates, I will focus on the merger history of black holes in gas-rich galaxy collisions to show how sensitive is the physics of the inspiral to the properties of the gaseous and stellar environment. N-Body/hydrodynamical codes have proven to be vital tools for studying the dynamics of the holes, and progress in this field is expected to grow rapidly in the effort to describe, in full realism, the physics of stars and gas around the black holes, starting from the large scale of a cosmic merger (hundreds of kpc) down to the scale of coalescence (a few astronomical units).