3.2 Galaxy cluster research: current perspectives
3.2.1 An X-ray view of galaxy clusters
The hot diffuse intra-cluster medium cools through a combination of thermal bremsstrahlung, recombination and de-excitation radiation giving rise to extended X-ray emission (Sarazin 1988, B¨ohringer et al. 2010). The X-ray luminosity of galaxy clusters, LX, ranges from 1043 to 1045 erg s−1. X-ray observations with high-resolution spatial and spectral instru- ments such as those on theChandra and XMM-Newton satellites, have enabled the study of density, metallicity and temperature profiles in galaxy clusters (Leccardi et al. 2008, Arnaud et al. 2010). The X-ray surface brightness of a cluster at redshift z , is given by
SX = 1
4π(1 +z)4 ,
n2eΛeHdl , (3.3)
where ΛeH is the X-ray cooling function and the integral is performed along the line of sight. The electron density squared dependence implies that X-ray observations are particularly sensitive to regions in which the density is highest, as well as to clumping effects in cluster outskirts and shock structures in the central regions of clusters (Fig.
3.2).
The Perseus cluster,z= 0.0179, being the brightest X-ray observed cluster in the sky, is a prime example for illustrating the potential of X-ray observations. Recent observations with the Suzaku satellite have allowed cluster outskirts to be mapped out to beyondR200 (Simionescu et al. 2011, Urban et al. 2014). Urban et al. (2014) explain the mismatch between measured and expected density and pressure profiles due to the effect of gas clumping (Fig. 3.2).
Fabian et al. (2003, 2011) and Sanders & Fabian (2007) have studied the centre of the Perseus cluster with deep exposure Chandra measurements (Fig. 3.2). The resulting high-resolution X-ray image shows a highly disturbed intra-cluster medium, classified by a combination of a cold front, a shock structure (Churazov et al. 2003), sound wave ripples as well as two X-ray cavities. The latter are caused by relativistic plasma that is blown into the ICM through AGN radio jets, observed with the VLA (Fabian et al. 2003). It is the low redshift of the Perseus cluster, which enables such detailed analyses to be feasible in terms of a cost benefit analysis considering scientific outcome versus the required observation time. Due to the 1/(1 +z)4 dependence of the X-ray surface brightness, it is not viable to invest an equivalent or even higher observation time on clusters at higher redshifts due to the decreased photon count statistics (Fig. 3.3).
3.2. Galaxy cluster research: current perspectives 23
Figure 3.2: X-ray observations of the Perseus clusters. Left: An image by Urban et al. (2014) showing Suzaku observations of the Perseus cluster beyond the virial radius (dashed circle), the small circles indicating the presence of point sources. Right: An image by Fabian et al. (2011) showing the adaptively smoothed fractional variation image of an 1.4 Ms Chandra exposure. One can clearly see the X-ray cavities and associated sound waves, illustrating the sensitivity of X-ray observations to density contrasts.
Figure 3.3: A mock simulations study taken from Santos et al. (2008), outlining the decreased photon count statistics at higher redshifts for clone clusters with the same properties as their lower-redshift counterpart A2163. A qualitative comparisons with real data at these redshifts is given below.
Figure 3.4: The Arnaud galaxy cluster sample illustrating relaxed and morphologically disturbed clusters. Top: Fabian & Sanders (2009) illustrate the difference in the nature of relaxed and morphologically disturbed galaxy clusters via the excess central surface brightness signature in the former, which also manifests itself in a higher central density profile(bottom left)and lower central temperature value(bottom right)as is illustrated by the thez<0.2 Arnaud cluster sample (Arnaud et al. 2010). The profiles are scaled toR500 and normalized by the respective mean density withinR500 and the average spectroscopic temperature within [0.15 - 0.75]R500 (R500 denotes the radius at which the density is 500 times the critical density at the cluster’s redshift).
3.2. Galaxy cluster research: current perspectives 25 Radial temperature information is usually obtained from spectral fitting in annular regions whose total areas are dependent on the photon count statistics.
In the early era of X-ray observations, Cavaliere & Fusco-Femiano (1976) put forward the isothermal beta profile. In galaxy cluster studies, this has been used extensively as a parametric description of galaxy clusters’ surface brightness radial variations, such that
SX =SX0 -
1 +θ2 θ2c
.12−3β
, (3.4)
where SX0 is the central surface brightness, β is the slope parameter and θc corresponds to the angular core radius. The corresponding expression for the gas density under the assumptions of spherical symmetry, isothermal cluster properties and hydrostatic equilib- rium gives
ng(r) =ng,0 /
1 +r2 rc2
0−3β2
, (3.5)
where ng,0 is the central gas density and rc is the core radius. With high resolution Chandra and XMM-Newton X-ray satellite missions opening up the opportunity of 1$$- 6$$ angular resolution observations, annular spectroscopic temperature fitting has become possible in the past decade. It was shown that the isothermal assumption was not valid for clusters since radial temperature dependences were indeed observed (Arnaud et al.
2010, Fig. 3.4). In addition, a sub-sample of clusters were shown to exhibit excess surface brightness signatures in the centre of clusters (Fig. 3.4 (top)).
These clusters were thus termed cool-core clusters and the concept of a double-beta model fit with inner and outer normalization, core and slope parameters was suggested and applied to data. This discovery also led to the ’cooling-flow’ problem on account of the low detected quantity of cold gas in the clusters’ central regions and the lower than expected star formation rate from sole mass deposition rate estimates (Peterson & Fabian 2006). At redshifts below 0.2, Arnaud et al. (2010) illustrate that, on average, cool-core clusters exhibit a decline in temperature towards their central regions and can thus be distinguished from less relaxed systems.
A detailed discussion on the derived characteristic pressure and entropy profiles, as well as the possible solutions to the ’cooling flow’ problem, will follow in section 3.4 and chapter 7.