Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-07-04
Astronomy and Astrophysics
Astrophysics
Accepted for publication by MNRAS, revised version to fit published version, now includes discussion of power-law mass distrib
Scientific paper
10.1046/j.1365-8711.2003.06131.x
A semi-analytic model of cluster cooling flows is presented. The model assumes that episodic nuclear activity followed by radiative cooling without mass-dropout cycles the cluster gas between a relatively homogeneous, nearly isothermal post-outburst state and a cuspy configuration in which a cooling catastrophe initiates the next nuclear outburst. Fitting the model to Chandra data for the Hydra cluster, a lower limit of $284\Myr$ until the next outburst of Hydra A is derived. Density, temperature and emission-measure profiles at several times prior to the cooling catastrophe are presented. It proves possible to fit the mass $M(\sigma)$ with entropy index $P\rho^{-\gamma}$ less than $\sigma$ to a simple power-law form, which is almost invariant as the cluster cools. We show that radiative cooling automatically establishes this power-law form if the entropy index was constant throughout the cluster gas at some early epoch or after an AGN activity cycle. To high precision, the central value of $\sigma$ decreases linearly in time. The fraction of clusters in a magnitude-limited sample that have gas cooler than $T$ is calculated, and is shown to be small for $T=2\keV$. Similarly, only 1 percent of clusters in such a sample contain gas with $P\rho^{-\gamma} < 2\keV\cm^2$. Entropy production in shocks is shown to be small. The entropy that is radiated from the cluster can be replaced if a few percent of the cluster gas passes through bubbles heated during an outburst of the AGN.
Binney James J.
Kaiser Christian R.
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