Entropy Injection as a Global Feedback Mechanism

Both pre-heating of the intergalactic medium and radiative cooling of low entropy gas have been proposed to explain the deviation from self-similarity in the cluster L X‐T X relation and the observed entropy floor in these systems. However, severe overcooling of gas in groups is necessary for radiative cooling alone to explain the observations. Non-gravitational entropy injection must therefore still be important in these systems. We point out that, on scales of groups and below, gas heated to the required entropy floor cannot cool in a Hubble time, regardless of its subsequent adiabatic compression. Pre-heating therefore shuts off the gas supply to galaxies, and should be an important global feedback mechanism for galaxy formation. Constraints on global gas cooling can be placed from the joint evolution of the comoving star formation rate and neutral gas density. Pre-heating at high redshift can be ruled out; however, the data do not rule out passive gas consumption without inflow as z ∼ 2. Because for pre-heated gas t cool > t dyn, we speculate that pre-heating could play a role in determining the Hubble sequence; at a given mass scale, high σ peaks in the density field collapse early to form ellipticals, while low σ peaks collapse late and quiescently accrete pre-heated gas to form spirals. The entropy produced by large-scale shock-heating of the intergalatic medium is significant only at late times, z < 1, and cannot produce these effects.