Dynamical-friction galaxy-gas coupling and cluster cooling flows

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We revisit the notion that galaxy motions can efficiently heat intergalactic gas in the central regions of clusters through dynamical friction. For plausible values of the galaxy mass-to-light ratio, the heating rate is comparable with the cooling rate due to X-ray emission. Heating occurs only for supersonic galaxy motions, so the mechanism is self-regulating: it becomes efficient only when the gas sound speed is smaller than the galaxy velocity dispersion. We illustrate with the Perseus cluster, assuming a stellar mass-to-light ratio for galaxies in the very central region with the dark matter contribution becoming comparable with this at some radius rs. For rs ≲ 400 kpc ∼ 3rcool - corresponding to an average mass-to-light ratio of ∼10 inside that radius - the dynamical-friction coupling is strong enough to provide the required rate of gas heating. Such values of rs are associated with total mass attached to galaxies that is about 10 per cent of the mass of the cluster - consistent with values inferred from numerical simulations and observations. The measured sound speed is smaller than the galaxy velocity dispersion, as required by this mechanism. With this smaller gas temperature and the observed distribution of galaxies and gas, the energy reservoir in galactic motions is sufficient to sustain the required heating rate for the lifetime of the cluster. The galaxies also lose a smaller amount of energy through dynamical friction to the dark matter implying that non-cooling-flow clusters should have flat-cored dark matter density distributions.

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