Long-Term Gravitational Quenching by Clumpy Accretion in Galaxies and Clusters

We consider a simple gravitational-heating mechanism for the long-term quenching of cooling flows and star formation in massive dark-matter haloes hosting elliptical galaxies and clusters. The virial shock heating in haloes > 10M⊙ provides natural quenching in 10M⊙ haloes (Birnboim et al. 2007). Analytic estimates and simple simulations argue that the long-term quenching in haloes > Mmin ∼ 7×10M⊙ could be due to the gravitational energy of cosmological accretion delivered to the inner-halo hot gas by cold gas clumps of ∼ 10M⊙ via ram-pressure drag and local shocks. Mmin is obtained by comparing the gravitational power of infall into the potential well with the overall radiative cooling rate. The heating wins if the gas inner density cusp is not steeper than r and if the masses in clumps and in ambient gas are comparable. The effect is stronger at higher redshifts, making the maintenance easier at all times. Clumps > 10M⊙ penetrate to the inner halo before they halt or disintegrate, but they have to be 6 10M⊙ for the drag to be effective in a Hubble time. Pressure confined ∼ 10K clumps are stable against their own gravity and remain gaseous once below the Bonnor-Ebert mass ∼ 10M⊙. Such clumps are also immune to tidal disruption. Clumps in the desired mass range could emerge by thermal instability in the outer halo or in the filaments that feed it if the conductivity is not too high. Alternatively, such clumps may be embedded in dark-matter subhaloes if the ionizing flux is ineffective, but they separate from their subhaloes by ram pressure before entering the inner halo. Heating by dynamical friction may become substantial only for rather massive satellites. We conclude that clumpy accretion is a viable alternative to AGN feedback as a long-term quenching mechanism.