Professor James Binney FRS

Abstract:

We determine the critical size l_crit of a filament of cold (T~10^4 K) gas that is in radiative equilibrium with X-ray emitting gas at temperatures T_out~10^6 - 10^8 K. Filaments smaller than l_crit will be rapidly evaporated, while longer ones will induce the condensation of the ambient medium. At fixed pressure P, l_crit increases as T_out^(11/4), while at fixed T_out it scales as 1/P. It scales as f^(1/2), where f is the factor by which the magnetic field depresses the thermal conductivity below Spitzer's benchmark value. For plausible values of f, l_crit is similar to the lengths of observed filaments. In a cluster such as Perseus, the value of l_crit increases by over an order of magnitude between the centre and a radius of 100 kpc. If the spectrum of seed filament lengths l is strongly falling with l, as is natural, then these results explain why filaments are only seen within a few kiloparsecs of the centres of clusters, and are not seen in clusters that have no cooling flow. We calculate the differential emission measure as a function of temperature for the interface between filaments and ambient gas of various temperatures. We discuss the implications of our results for the origin of the galaxy luminosity function.

Abstract:

Three-dimensional hydrodynamical simulations are used to investigate the structural stability of cooling flows that are episodically heated by jets from a central AGN. The radial profile of energy deposition is controlled by (a) the power of the jets, and (b) the pre-outburst density profile. A delay in the ignition of the jets causes more powerful jets to impact on a more centrally concentrated medium. The net effect is a sufficient increase in the central concentration of energy deposition to cause the post-outburst density profile to be less centrally concentrated than that of an identical cluster in which the outburst happened earlier and was weaker. These results suggest that the density profiles of cooling flows oscillate around an attracting profile, thus explaining why cooling flows are observed to have similar density profiles. The possibility is raised that powerful FR II systems are ones in which this feedback mechanism has broken down and a runaway growth of the source parameters has occurred.

Abstract:

An estimate of the convergence radius of a simulated CDM halo is obtained under the assumption that the peak phase-space density in the system is set by discreteness effects that operate prior to relaxation. The predicted convergence radii are approximately a factor 2 larger than those estimated for numerical convergence studies. A toy model is used to study the formation of sheets of the cosmic web, from which DM haloes form later. This model demonstrates the interplay between phase mixing and violent relaxation that must also be characteristic of spherical collapse. In the limit that sheets contain arbitrarily many particles, it seems that power-law profiles are established in both distance and energy. When only a finite number of particles is employed, relaxation is prematurely terminated and the power laws are broken. In a given simulation, the sheets with the highest peak phase-space densities are those that form from the longest waves. Hence simulations with little small-scale power are expected to form the cuspiest haloes.