Color shows mean stellar age, from 10⁷ yr (blue) to ~10⁹ yr (red). Brightness shows the projected stellar surface density, scaled logarithmically.
This movie shows typical simulations of major galaxy mergers. In the mergers, two Milky Way-like disk galaxies with a 2:1 mass ratio are placed in an initial parabolic orbit. The disks are constructed in equilibrium, with properties corresponding to observed disk galaxies, and include stellar and gas disks, central spheroidal bulges, and black holes, within a dark matter halo. As the halos orbit, the galaxies lose angular momentum to the dark matter via dynamical friction and sink to the center. Upon first passage, torques exerted by the systems drive gas to the centers of each galaxy, which in turn begins to feed black hole growth and central star formation. The large gas densities and small relative size of the black hole ensure it remains shrouded in gas and dust, as a classical ULIRG. This builds up and the starburst eventually peaks as the galaxies coalesce. The black holes of both galaxies rapidly sink to the center of the galaxy and merge, growing until, shortly after the final coalescence, the black hole is sufficiently massive and luminous that the coupling of even a small fraction of the radiated accretion energy to the surrounding gas rapidly unbinds and heats that gas. The small amount of remaining cold gas is driven out in an explosive, quasar-driven wind, leaving a black hole that obeys observed correlations between black hole and host galaxy mass. With star formation effectively terminated, the remnant then gradually relaxes and its colors redden, as it evolves to resemble a typical, local massive elliptical galaxy.
credit: Philip Hopkins