Quasar Feedback: The Missing Link in Structure Formation

We consider the impact of quasar outflows on structure formation. Suchoutflows are potentially more important than galactic winds, which appearinsufficient to produce the level of preheating inferred from X-rayobservations of galaxy clusters. Using a simple analytical model for thedistribution of quasars with redshift, coupled with a one-dimensionalSedov-Taylor model for outflows, we are able to make robust statements abouttheir impact of on structure formation. As large regions of the IGM are heatedabove a critical entropy of approximately 100 keV cm^2, cooling becomeimpossible within them, regardless of changes in density. On quasar scales,this has the effect of inhibiting further formation, resulting in the observedfall-off in their number densities below z = 2. On galaxy scales, quasarfeedback fixes the turn-over scale in the galaxy luminosity function (L_*) asthe nonlinear scale at the redshift of strong feedback. The galaxy luminosityfunction then remains largely fixed after this epoch, consistent with recentobservations and in contrast to the strong evolution predicted in more standardgalaxy-formation models. Finally, strong quasar feedback explains why theintracluster medium is observed to have been pre-heated to entropy levels justabove S_crit, the minimum excess that would not have been erased by cooling.The presence of such outflows is completely consistent with the observedproperties of the Lyman-alpha forest at z ~ 2, but is expected to have asubstantial and detectable impact on Compton distortions observed in themicrowave background and the multiphase properties of the "warm-hot" (z=0)circumgalactic medium.Comment: 20 pages, 11 figures, ApJ, accepted (minor changes made, two plots updated