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Feedback heating from active galactic nuclei (AGNs) has been commonly invokedto suppress cooling flows predicted in hot gas in elliptical galaxies, galaxygroups and clusters. Previous studies have focused on if and how AGN feedbackheats the gas, but little paid attention to its triggering mechanism. Usingspherically symmetric simulations, we investigate how large-scale cooling flowsare accreted by central supermassive black holes (SMBHs) in eight well-observedsystems and find an interesting dichotomy. In massive clusters, the gasdevelops a central cooling catastrophe within about the cooling time (typically~ 100 - 300 Myr), resulting in a cold-mode accretion onto SMBHs. However, inour four simulated systems on group and galaxy scales at a low metallicityZ=0.3Z_{sun}, the gas quickly settles into a long-term state which has a cuspycentral temperature profile extending to several tens to about 100 pc. At themore realistic solar metallicity, two groups (with R_{e} ~ 4 kpc) still hostthe long-term hot-mode accretion. Both accretion modes naturally appear in ouridealized calculations where only cooling, gas inflow, and compressionalheating are considered. The long-term hot-mode accretion is maintained by thequickly-established closeness between the timescales of these processes,preferably in systems with low gas densities, low gas metallicities, andimportantly, compact central galaxies, which result in strong gravitationalacceleration and compressional heating at the intermediate radii. Ourcalculations predict that central cuspy temperature profiles appear more oftenin smaller systems than galaxy clusters, which instead often host significantcold gas and star formation.

astrophysical journal/the astrophysical journal

Hot versus Cold: the Dichotomy in Spherical Accretion of Cooling Flows  onto Supermassive Black Holes in Elliptical Galaxies, Galaxy Groups and  Clusters