Merging Clusters of Galaxies

In the standard model of hierarchical growth of large-scale structure, where small mass aggregates collapse and form first, galaxy clusters grow by the inhomoge-neous accretion of matter. Mass units of various mass fall into the cluster potential and if these mass units are large enough they are observed as cluster mergers. Major mergers of massive clusters, that is mergers with subunits of comparable mass, happen with collision velocities of the order of 2000 km s-1 and are with a release of gravitational energy of as much as >1064erg, the most energetic events in the Universe next to the Big Bang [56]. At their collision velocities shocks are created in the merging ICM, which dissipate much of the gravitational energy. It is these shocks that form the major source of thermal energy due to which we observe these extremely high ICM temperatures.

An excellent overview on the astrophysics and observations of galaxy cluster mergers is given in a recent book by Feretti et al. [56]. The large amount of energy dissipated in cluster mergers make them very interesting events for further studies, as for example for the physics of shock heating. The radio halos observed in galaxy clusters are presumably created by cosmic ray acceleration in merger shocks and turbulence [67], which also boosts the strength of the intracluster magnetic fields.

Figure 23.8, shows the results of detailed XMM studies of the two most dramatic major mergers in nearby galaxy clusters. For the cluster A754 the Figure shows the image of the X-ray surface brightness [73] with contours of the radio halo observed at 20 cm [5] superposed. The system is interpreted as a merger of subclusters where core passage happened on the order of half a Gyr ago. The centers of the two sub-clusters are still noticeable, with the cool, dense core of the main cluster seen now in the South-East and the center of the other subcluster at the X-ray surface brightness maximum in the North-West. The radio halo is found in the region of highest pressure and probably the zone of largest turbulence. The shocks and the turbulence are most probably the sites of cosmic ray acceleration, which produces the relativistic electrons observed through their synchrotron radio radiation.

The right panel in Fig. 23.8, shows the temperature map of the merging cluster A3667 [25]. Again we see a postmerger stage with a lot of turbulence clearly displayed by the temperature map. This cluster also host the most prominent radio relics, large radio structures on opposite ends in the cluster along the merging direction which are probably created by the outgoing merger shock [67]. Observations of several of such systems, where X-ray observations indicate a cluster merger and radio observations show a radio halo inferring an intense relativistic electron population let to the expectation that radio halos should generally be expected in massive,

Fig. 23.8 XMM-Newton observations of two of the most dramatic nearby merging clusters. Left: Surface brightness distribution map of the galaxy cluster A754 (Henry et al. [73]) with radio contours overlaid (Bacchi et al. [5]). The two surface brightness maxima mark the centers of the two merging subclusters that have most probably already passed the center. Right: Temperature map of the cluster A3667, where red and white regions are hot and blue and green regions are cool (Briel et al. [25]). This system is also interpreted as a major cluster merger after core passage. The central region between the centers of the merging subclusters shows a high degree of complexity and turbulence

Fig. 23.8 XMM-Newton observations of two of the most dramatic nearby merging clusters. Left: Surface brightness distribution map of the galaxy cluster A754 (Henry et al. [73]) with radio contours overlaid (Bacchi et al. [5]). The two surface brightness maxima mark the centers of the two merging subclusters that have most probably already passed the center. Right: Temperature map of the cluster A3667, where red and white regions are hot and blue and green regions are cool (Briel et al. [25]). This system is also interpreted as a major cluster merger after core passage. The central region between the centers of the merging subclusters shows a high degree of complexity and turbulence merging clusters (e.g. [57]). A statistical investigation of cluster substructure and radio halo occurrence supports this scenario [130].

To directly observe and study shock fronts in major mergers has turned out to be quite difficult and only in very few cases have shocks been observed clearly. One of the best examples is found in the cluster E0657-56 (RXCJ0568-5557) [62,92] and is shown in Fig. 23.9. While the high resolution surface brightness image obtained with Chandra shows a bow shock type feature with a Mach cone [92], the X-ray color and spectral analysis of the cluster data reveal a region of very high entropy in front of the bow shock like feature [62]. This region of high entropy is a direct manifestation of the heating and entropy production of merger shocks.

Theoretical considerations and simulations also predict a large degree of turbulence in post-merger galaxy clusters [77,143]. That the ICM of a postmerger cluster has the expected structure of evolved Kolmogorov-Obuchov turbulence could recently be shown with a deep study of the Coma cluster with XMM-Newton [134]. Traditionally the structure of evolved turbulence is characterized by a spatially correlated velocity field, which shows a power law power spectrum [88]. This power law behavior of the spatial correlation is also expected for the pressure distribution (with a different value for the exponent). The pressure distribution of the central part of the Coma cluster, believed to be a postmerger system [159], is shown in Fig. 23.10 [134]. The pressure values for the pixels of the image have been determined from the surface brightness and spectral hardness ratio, the two observables from which the ICM density and temperature can be deduced. An analysis of the pressure distribution shows a power law power spectrum of the kind expected for

Fig. 23.9 Left: Chandra image of the merging cluster E0657-56 (RXCJ0568-5557) [92]. The bright feature on the upper right with a Mach cone like, sharp surface brightness structure is the core of a subcluster flying at high speed ("bullet") through the main cluster. Right: Projected temperature (upper left), entropy (upper right), pressure (lower left), and surface brightness map of the cluster E0657-56 (RXCJ0568-5557) constructed from XMM-Newton imaging and spectroscopic data [62] (for details of the construction of these maps see [73]). Most interesting is the high entropy feature seen in the entropy map in front of the "bullet" (which itself features a very low entropy unveiling itself as a cooling core with a cold front). This high entropy structure is most probably caused by shock heating

Fig. 23.9 Left: Chandra image of the merging cluster E0657-56 (RXCJ0568-5557) [92]. The bright feature on the upper right with a Mach cone like, sharp surface brightness structure is the core of a subcluster flying at high speed ("bullet") through the main cluster. Right: Projected temperature (upper left), entropy (upper right), pressure (lower left), and surface brightness map of the cluster E0657-56 (RXCJ0568-5557) constructed from XMM-Newton imaging and spectroscopic data [62] (for details of the construction of these maps see [73]). Most interesting is the high entropy feature seen in the entropy map in front of the "bullet" (which itself features a very low entropy unveiling itself as a cooling core with a cold front). This high entropy structure is most probably caused by shock heating

Fig. 23.10 Image of the projected pressure distribution in the COMA cluster constructed from XMM-Newton imaging and spectroscopic data by Schuecker et al. [134]. The scale of 145 kpc indicated in the figure corresponds to the largest size of turbulent eddies revealed by the turbulence power spectrum obtained from this image [134]

Fig. 23.10 Image of the projected pressure distribution in the COMA cluster constructed from XMM-Newton imaging and spectroscopic data by Schuecker et al. [134]. The scale of 145 kpc indicated in the figure corresponds to the largest size of turbulent eddies revealed by the turbulence power spectrum obtained from this image [134]

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Fig. 23.11 Left: Correlation of the density and temperature fluctuations in the central region of the ICM of the Coma cluster of galaxies [134]. The correlation diagram is obtained by running a sliding window through the data thus that not all data points are independent. The resulting correlation slope implies that the fluctuations are close to adiabatic. Right: Power spectrum of the pressure fluctuations in the turbulent central region of the ICM of the Coma cluster [134]

Fig. 23.11 Left: Correlation of the density and temperature fluctuations in the central region of the ICM of the Coma cluster of galaxies [134]. The correlation diagram is obtained by running a sliding window through the data thus that not all data points are independent. The resulting correlation slope implies that the fluctuations are close to adiabatic. Right: Power spectrum of the pressure fluctuations in the turbulent central region of the ICM of the Coma cluster [134]

Kologorov-Oboukhov-turbulence as shown in Fig. 23.11 (right). Further support for the interpretation of the observed features as classical turbulence are (1) a Gaussian distribution of the pressure fluctuations, and (2) the fact that the pressure fluctuations show on average roughly a correlation of temperature and density of the form An/n « AT/T4/3, which is close to the adiabatic value of 5/3. The latter indicates that the fluctuations also seen in the surface brightness map are neither dominated by contact discontinuities ("cold fronts") nor by strong shocks, but rather by nearly adiabatic pressure waves [134]. Power law power spectra indicating developed turbulence in the ICM have also been seen in the orientation of the ICM magnetic field as traced by Faraday rotation measurements [155].

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