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We present a spherically symmetric model for the origin and evolution of thetemperature profiles in the hot plasma filling galaxy groups and clusters. Wefind that the gas in clusters is generically not isothermal, and that thetemperature declines with radius at large distances from the cluster center(outside the core- and scale radii). This temperature profile is determined bythe accretion history of the halo, and is not quantitatively well-described bya polytropic model. We explain quantitatively how the large-scale temperaturegradient persists in spite of thermal conduction and convection. These resultsare a consequence of the cosmological assembly of clusters and cannot bereproduced with non-cosmological simulations of isolated halos. We show thatthe variation in halo assembly histories produces a ~10% scatter in temperatureat fixed mass. On top of this scatter, conduction decreases the temperature ofthe gas near the scale radius in massive clusters, which may bias hydrostaticmass estimates inferred from x-ray and SZ observations. As an exampleapplication of our model profiles, we use mixing-length theory to estimate theturbulent pressure support created by the magnetothermal instability (MTI): inagreement with our earlier MHD simulations, we find that the convectionproduced by the MTI can provide ~5% non-thermal pressure support near r_500.The magnitude of this turbulent pressure support is likely to be non-monotonicin halo mass, peaking in ~10^14.5 M_sun halos.

What Sets Temperature Gradients in Galaxy Clusters? Implications for non-thermal pressure support and mass-observable scaling relations