Rapidly varying accretion and AGN feedback |
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Authors: | Edward C. D. Pope |
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Affiliation: | School of Physics &Astronomy, University of Leeds, Leeds LS2 9JT |
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Abstract: | Accretion rates on to active galactic nuclei (AGNs) are likely to be extremely variable on short time-scales; much shorter than the typical cooling time of X-ray emitting gas in elliptical galaxies and galaxy clusters. Using the Langevin approach it is shown that, for a simple feedback system, this can induce variability in the AGN power output that is of much larger amplitude, and persists for longer time-scales, than the initial fluctuations. An implication of this is that rich galaxy clusters are expected to show the largest and longest-lived fluctuations. Stochastic variations in the accretion rate also mean that the AGN injects energy across a wide range of time-scales. This allows the AGN to maintain a much closer balance with its surroundings than if it was periodically activated. The possible non-linear correlation between Bondi accretion rate and jet power, found by Allen et al., can be explained if the instantaneous accretion rate, scaled by jet power, varies log-normally. This explanation also implies that the duty cycle of AGN activity increases with the radiative losses of the surroundings, in qualitative agreement with Best et al. |
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Keywords: | galaxies: active galaxies: cooling flows galaxies: jets |
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