Time-dependent circulation flows: Iron enrichment in cooling flows with heated return flows

We describe a new type of dynamical model for hot gas in galaxy groups and clusters in which gas moves simultaneously in both radial directions. The observational motivations for this type of flow are compelling. X-ray spectra indicate that little or no gas is cooling to low temperatures. Bubbles of hot gas typically appear in Chandra X-ray images and XMM-Newton X-ray spectra within similar to 50 kpc of the central elliptical galaxy. These bubbles must be buoyant. Furthermore, the elemental composition and total mass of gas-phase iron observed within similar to 100 kpc of the center can be understood as the accumulated outflow of most or all of the iron produced by Type Ia supernovae in the central galaxy over time. This gaseous iron has been circulating for many gigayears, unable to cool. As dense inflowing gas cools, it produces a positive central temperature gradient, a characteristic feature of normal cooling flows. This gas dominates the local X-ray spectrum but shares the total available volume with centrally heated, outflowing gas. Circulating flows eventually cool catastrophically if the outflowing gas transports mass but no heat; to maintain the circulation both mass and energy must be supplied to the inflowing gas over a large volume, extending to the cooling radius. The rapid radial recirculation of gas within similar to 50 kpc results in a flat core in the gas iron abundance, similar to many group and cluster observations. We believe the circulation flows described here are the first gasdynamic, long-term evolutionary models that are in good agreement with all essential features observed in the hot gas: little or no gas cools as required by XMM spectra, the gas temperature increases outward near the center, and the gaseous iron abundance is about solar near the center and decreases outward.