Gravitational Heating Helps Make Massive Galaxies Red and Dead
  We study the thermal formation history of four simulated galaxies that were
shown in Naab et al. (2007) 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, lowering the central dark matter density by up to a
factor of two from z=3 to z=0. In galaxies in which the late formation history
(z<2) is dominated by minor merging and accretion the energy released
(E~5x10^{59} ergs) from gravitational feedback is sufficient to form red and
dead elliptical galaxies by z~1 even in the absence of supernova and AGN
feedback.
