AGN-ICM interaction in nearby cool core clusters: energetics and transport processes
Abstract
This work focuses on detailed spatial and spectral analysis of the properties of the intra-cluster medium (ICM) in clusters of galaxies, giant laboratories which allow us to probe the formation and chemical enrichment history of the Universe. These are the only objects large enough to contain a fair sample of all types of baryonic and dark matter in the Universe. The largest amount of baryons (about 80%) resides in a diuse ICM which is shock-heated during mergers associated with hierarchical large-scale structure formation to temperatures of 10^7 - 10^8 K and consequently emits mostly in the X-ray domain. This plasma permeates the entire cluster, tracing the gravitational potential dominated by the dark matter. The deep potential wells of clusters of galaxies retain all the metals produced by supernovae in the member galaxies throughout the cluster's life, therefore metal abundances in the ICM constitute a fossil record of the average chemical enrichment history of the Universe. Energetic interaction between the active galactic nucleus (AGN) in the brightest cluster galaxy (BCG) and the ICM is needed to heat the gas at some cluster centers where the surface brightness profile is very peaked. The energy radiated away here is so large that, if it came from thermal energy alone, the hot X-ray gas would have to cool and form copious amounts of stars, in disagreement with observations (the so-called "cooling flow" problem). Turbulence and gas motions induced by the AGN are moreover believed to be a main ingredient for transporting and distributing the metals within the ICM. I investigate features associated with AGN-ICM interaction in two bright and relatively nearby systems. M87, at the center of Virgo, the nearest galaxy cluster, is a natural choice as a first target for such detailed studies. Since it is so close, M87 is both bright enough to ensure excellent spectral statistics and enables us to resolve much smaller spatial scales than possible for any other object. The next most interesting case, apart from the Perseus cluster which had already been looked at in detail, is Hydra A. This cluster can be considered a scaled-up version of M87, where the power and scales involved in the AGN-ICM interaction are about an order of magnitude larger. In these two clusters, we find and map the first two known classical AGN-driven shocks with spectroscopically confirmed temperature and pressure jumps corresponding to consistent Mach numbers. These shocks are thought to be one main mechanism through which the AGN heats the ICM to prevent catastrophic cooling. I present both simplified 1D and 3D hydrodynamic simulations of the large-scale shock in Hydra A and compare the results with observations in order to estimate the Mach number and energy of the shock. The 3D simulations include a bulk flow of the ICM as a possible explanation for the asymmetry of the observed shock front shape and its offset with respect with the cluster center. In M87, I show a cold front that suggests the presence of bulk motions in the form of sloshing in the ICM. Cool, metal-rich filaments, which can often be resolved into multi-temperature components and are spatially coincident with AGN-inflated radio lobes, suggest moreover the crucial role of the AGN in uplifting the chemical elements produced by the central galaxy and transporting them into the ICM in both clusters. In M87, where the statistics allow detailed modeling of the temperature structure on small spatial scales, I find a correlation between the amount of cool gas and the metal abundance (averaged over all temperature phases), based on which I deduce the metallicity of the cool gas. This gas is cooler compared to the surrounding ICM most probably because of adiabatic expansion, as it was dragged by the rising radio lobes towards larger radii where the ambient pressure is smaller; it is more metal-abundant because it was enriched by supernovae and stellar mass loss at the center of the galaxy. I estimate the gas mass, Fe mass and energy associated with the uplift for both clusters. I find that the AGN can uplift a sizable amount of Fe compared to the mass of Fe currently present in the cluster center. I compare the energies related with gas uplift with those needed to create the AGN-driven shocks and AGN-inflated bubbles and show that, for M87, the energies required to produce these three substructures are similar. In Hydra A, the energy needed to uplift the gas is similar to that associated with the most recently inflated bubbles, but orders of magnitude smaller than the energy of the large-scale shock created during a much more powerful past outburst. I moreover discuss the timescales in which supernovae and stellar winds can produce the observed amount of metals and metal abundance ratios, and compare the observed metal abundance ratios with models of heavy element production by dierent supernova types.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2009
- Bibcode:
- 2009PhDT.......468S