GRAVITATIONAL HEATING HELPS MAKE MASSIVE GALAXIES RED AND DEAD

We study the thermal formation history of four simulated galaxies that were shown by Naab et al. to reproduce a number of observed properties of elliptical galaxies. The temperature of the gas in the galaxies is steadily increasing with decreasing redshift, although much of the gas has a cooling time shorter than the Hubble time. The gas is being heated and kept hot by gravitational heating processes through the release of potential energy from infalling stellar clumps. The energy is dissipated in supersonic collisions of infalling gas lumps with the ambient gas and through the dynamical capturing of satellite systems causing gravitational wakes that transfer energy to the surrounding gas. Furthermore, dynamical friction from the infalling clumps pushes out dark matter (DM), lowering the central DM density by up to a factor of 2 from z = 3 to z = 0. In galaxies in which the late formation history (z less than or similar to 2) is dominated by minor merging and accretion, the energy released (E similar to 5 x 10(59) erg) from gravitational feedback is sufficient to form red and dead elliptical galaxies by z similar to 1 even in the absence of supernova and AGN feedback.