Entrevistas

We analyzed a deep XMM-Newton observation of the cluster of galaxies Hydra A and focused on the large-scale shock discovered as a surface brightness discontinuity in Chandra images (Nulsen

5.9 Summary and Outlook 119

et al. 2005). We find

• that the shock front can be seen in the pressure map as a 20% enhancement with respect to the radial average.

• that the shape of the shock seen in the pressure map can be approximated with an ellipse with a semi-major axis of 360 kpc oriented 10 degrees clockwise from the N–S direction, a semi-minor axis of 275 kpc, and centered ∼70 kpc towards the NE with respect to the cluster center. This is a good simple approximation to the shock shape seen in the Chandra image, which shows however some more complex deviations from ellipticity.

• for the first time, indications of temperature jumps corresponding to the shocked regions. We divided the data in four sectors towards the N, E, S and W and find temperature jumps with typical significances of 2σ. Combining the significances in the individual sectors we obtain a total significance of 4.3σ.

We then used a spherically symmetric hydrodynamic model of a point explosion at the center of an initially isothermal, hydrostatic atmosphere (Nulsen et al. 2005) to simulate surface bright- ness profiles and temperature jumps across the shock. These were compared to the observational data to estimate the shock properties, such as Mach number, energy and age. We find

• that the Mach numbers determined from the temperature jumps in the shocked regions are in good agreement with the Mach numbers derived from EPIC/pn surface brightness profiles and previously from Chandra data (Nulsen et al. 2005). This confirms that the large-scale surface brightness discontinuity in Hydra A is due to a classical shock.

• that the shock in all the four sectors has a Mach number consistent with∼1.3, although the distance between the shock front and the cluster center differs. This is contrary to what we would expect from a point explosion.

• estimated shock ages between 130 and 230 Myr. The larger shock age in the sectors where the shock is further from the cluster center suggests again that the shock generation mechanism is more complex.

• estimated shock energies between 1.5 and 3×1061ergs.

To further improve the modeling of the shock in Hydra A, we also employed 3D hydrody- namical simulations in which the shock is produced by a symmetrical pair of jets that originate from the cluster center, mimicking AGN activity. This creates an approximately elliptical shock front. To reproduce the observed 70 kpc offset between the cluster center and the center of the el- lipse approximation to the shock front, we included large-scale coherent motions in the simulated ICM. We find

• that the simulation can successfully reproduce the size, ellipticity and average Mach num- ber of the observed shock front.

• that the variation of the Mach number along the simulated shock is small, although the shock is asymmetric. This is in good agreement with the observed properties and could not be explained with a simple point explosion model.

• that the shock age and energy from the 3D simulation are 160 Myr and 3×1061 _{ergs, re-}

spectively, within the range of the estimated values based on the 1D shock model.

• that for the case of a potential flow around the central 100 kpc, the flow velocity needed to reproduce the observed offset of the center of the shock ellipse with respect to the cluster center is very large, 670 km s−1.

• that the AGN activity significantly broadens the temperature distribution in the cluster core. However, such a high bulk flow velocity coherent over large regions in the ICM is unlikely, and the simulation does not reproduce the proximity of the observed northern radio lobe to the shock front, which is potentially an important additional factor contributing to the offset of the shock ellipse. The morphology of the radio lobes, especially the bending of the southern lobe, is also difficult to reproduce, suggesting the necessity for more detailed simulations. In an upcom- ing paper, we plan to further investigate the shock modeling using more complex initial condi- tions, such as an elliptical cluster potential and the presence of older bubbles through which the shock propagates. We will also vary the jet physics to include intermittent activity. Switching the jet on and offwith different frequencies could put more momentum into bubbles, so that the shock becomes detached from them at a later stage. Moreover, the physics of the bubbles is im- portant: we will check the effect of including sub-grid turbulence models which should prevent the shredding of the bubbles (see Scannapieco & Br¨uggen 2008), making them easier to bend and be affected by bulk flow motions. This should significantly alleviate the problems described above and provide a more realistic model.

Acknowledgements

We acknowledge the support by the DFG grant BR 2026/3 within the Priority Programme “Wit- nesses of Cosmic History”, the supercomputing grants NIC 2195, 2256 and 2877 at the John- Neumann Institut at the Forschungszentrum J¨ulich, and NASA grant NNX07AQ18G 16610022. AS would like to thank the Harvard-Smithsonian CfA and Jacobs University Bremen for their hospitality. AF acknowledges support from BMBF/DLR under grant 50 OR 0207 and MPG. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). The Netherlands Institute for Space Research (SRON) is supported financially by NWO, the Nether- lands Organization for Scientific Research. Part of the results presented were produced using the FLASH code, a product of the DOE ASC/Alliances-funded Center for Astrophysical Thermonu- clear Flashes at the University of Chicago.

Chapter 6

Conclusions

6.1

Mechanisms and energetics of AGN-ICM interaction

The interaction between AGN and their environments is currently believed to be the key to un- derstanding galaxy and structure formation in detail. Both the formation of very massive galaxies and the distribution of the hot X-ray emitting ICM at the center of clusters of galaxies seem to be affected by AGN feedback. While the most common signatures of AGN-ICM interaction in clusters of galaxies are typically in the form of X-ray cavities filled with radio plasma injected by the AGN, I focused in this thesis on two other types of substructure features in the X-ray gas generated by the supermassive black holes in the BCGs: bright filaments dragged in the wake of buoyantly rising radio lobes and weak shocks. As the results summarized in Table 6.1 show, these two mechanisms contribute significantly to heating the central ICM.

While keeping in mind that the observed substructure is often a superposition of several AGN outbursts, some during which more energy may be deposited by one particular mechanism than another, it is worthwhile noting that

• the energy in the AGN-induced weak shocks is comparable to and typically slightly higher than the heat provided by all the observed buoyant X-ray cavities combined. The shocks in M87 and Hydra A are to date the only AGN-driven shocks where temperature and density discontinuities consistent with classical Rankine-Hugoniot jump conditions have been confirmed (Chapters 2 and 5).

• the typical energies associated with AGN-ICM interaction differ by as much as 4 orders of magnitude between the two clusters investigated here. This is partly due to the different energy required to quench the cooling (which is directly proportional to the nominal mass deposition rate shown in Table 6.1, the gas mass which should cool in the absence of heating) and partly to how often AGN outbursts occur. The cooling of the central ICM can be offset either by very energetic events separated by longer time intervals (potentially the case of Hydra A) or by less energetic outbursts occurring more frequently.

• the ratio of the energy deposited in weak shocks vs. cavities is similar for the two in- vestigated clusters. The energy needed to uplift the observed X-ray bright filaments on

Table 6.1: Comparison of energies associated with various AGN-ICM interaction mechanisms in M87 and Hydra A.

Interaction mechanism M87 Hydra A

X-ray cavities (total) 2.1−3.9×1057_ergsa ₀.₈₋₁._6×1061_ergsb

Filaments 4.3×1057ergs (Chapter 3) 1.3×1059ergs (Chapter 4)

Weak shock 8×1057 _ergsc _3×1061 _{ergs (Chapter 5)}

Nominal mass deposition rate 10 M/yr (Stewart et al. 1984) 300 M/yr (David et al. 2001)

a_{(Forman et al. 2005, 2007)} b_{(Wise et al. 2007)}

c_{(Forman et al. 2005)}

the other hand is comparable to the weak shock energy in M87 while being two orders of magnitude less than the weak shock energy in Hydra A. Thus, either the outburst energy in different clusters is not distributed in the same relative amounts between the three different mechanisms discussed here, or the uplifted filaments are a much more fragile feature. The energy of only the most recent, smallest cavities in Hydra A is only 6.4×1059_{ergs (Wise}

et al. 2007), comparable to that associated with the filaments. We could thus be seeing only the filaments associated with the latest outburst, while those associated with the previous, more energetic one, which created the larger outer bubbles, have been destroyed and mixed into the ICM and are no longer observed.

The conclusion to be drawn from this section is therefore that AGN do provide enough energy to prevent catastrophic cooling of the central ICM for tens to hundreds of megayears, but there are several mechanisms which all contribute significantly to depositing this energy into the ICM. One fourth possibility debated at the moment, apart from the three mechanisms presented in Table 6.1, are sound waves induced by the AGN which travel through the cluster towards the outskirts also depositing energy into the ICM (e.g. Sanders & Fabian 2007). These sound waves have not been observed in either of the two clusters presented here. To progress in our future understanding of AGN feedback therefore, one main goal will be to investigate more closely which of the different AGN-ICM interaction mechanisms observed so far is responsible for what fraction of the total energy budget, and if and under which conditions these relative contributions vary.