This final section will present a short description of different independent cosmological tests that use galaxy groups and clusters as probes, and which have been used successfully during the last decade and will be of important relevance in the near future with upcoming galaxy cluster surveys and new observatories. As mentioned before, the use of distinct methods allows a cross-check of the current cosmological model with procedures that have a different physical origin. The review from Allen et al. (2011) presents an extensive description of such techniques.
1. Cluster (Halo) mass function. This method has been fully described in Section2.2.3. Briefly, by measuring the number density of galaxy groups and clusters of a given mass and at certain redshift, one can obtain constraints mainly on the matter density,Ωm, and the amplitude of the density fluctuations, σ8. At the same time, its redshift evolution constrains the linear growth of the density perturbations, which depends also onΩm and the dark energy density,ΩΛ. This test requires a precise determination of the masses of the systems, as well as their redshift.
2. Clustering properties. The clustering refers to the measurement of the correlation function of the large-scale distribution of galaxy groups and clusters. It provides constraints on the matter and dark energy densities,Ωm andΩΛ.
Figure 2.7:Joint 68.3% and 95.4% confidence regions for the dark energy equation of state, w, and the matter density,Ωm, obtained from galaxy cluster abundance (purple regions) and gas mass fraction measurements, fgas
(red regions). Both methods are derived directly from galaxy clusters, and are compare with other cosmological test: CMB power spectrum (blue regions), supernova light curve measurements (green regions), and baryon acoustic oscillations from galaxy clustering (yellow regions). The combination of all methods gives the confidence regions in orange. Figure adapted from Allen et al. (2011).
3. Matter power spectrum. The amplitude and shape of the matter power spectrum (Eq. 2.32) depends also on the matter density, Ωm, and the amplitude of the density fluctuations, σ8. This test requires surveys of large area coverage, especially all-sky surveys are suitable for this method. The matter power spectrum and the correlation functions are two different ways of measuring the same properties since they are related by the Fourier transform.
4. Baryonic acoustic oscillations (BAO). BAOs are regular, periodic fluctuations in the density of the visible baryonic matter of the Universe. They were created by reciprocal settlement of baryons and dark matter into each other’s potentials. They are measured through the ‘baryonic wiggles’ of the power spectrum. BAOs can constrain the dark energy density,ΩΛ, angular diameter distance, DA, and trace the expansion history of the Universe through H(z). This requires very large samples of galaxy clusters (tens of thousands).
5. Gas mass fraction. This test is based on the assumption that galaxy clusters are representative objects of the matter content in the Universe. This is measured through the total baryon fraction fb = fgas+ fstars, i.e. fb is the sum of the gas mass and star mass fraction. The former can be measured through X-ray observations and the latter through optical. Therefore, the total baryon fraction in the outskirts of galaxy cluster should approach to the cosmic value. This test is sensitive to the matter density, sinceΩm = Ωb/ fb. Furthermore, assuming that fgas≈const at all redshifts, it can be used as a ‘standard ruler’ to probe the global geometry of the Universe, being also sensitive to the dark energy density,ΩΛ, angular diameter distance, DA, and the equation of state, w.
6. Absolute distance measurements. By combining X-ray and SZ observations of a galaxy cluster, its absolute distance can be determined, which allows the measurement of the Hubble constant,
2.2 Galaxy groups and clusters as cosmological probes
H0. From Eqs. 2.6 and 2.10 one has SX ∝ R n2eDAdθ and y ∝ neTedl ∝ neTeDAdθ, where dl = DAdθ, where θ is the line of sight angular size. By eliminating ne from both equations one obtain DA ∝ ∆TCMB2 /SXTe2 ∝ cz/H0. The results from this test are compatible with the DA determination from supernovae Ia studies.
7. SZ power spectrum. The thermal SZ effect originates in the ICM of galaxy groups and clusters, which causes a fluctuation in the CMB temperature at small angular scales. Therefore, it is ex- pected that the whole population of galaxy groups and clusters also generate a distortion, which is statistically described by its power spectrum. The distortion signal comes from all galaxy clusters even if they are not individually detected. In this sense, the tSZ power spectrum does not need the measurement of the mass of individual galaxy clusters and is insensitive to observational se- lection effects. This method is highly sensitive to the matter density, Ωm, and the amplitude of the density fluctuations, σ8. A number of recent experiments have measured the SZ power spectrum amplitude (e.g. Reichardt et al.2012), which proved to be lower than model predictions. The con- straints on σ8obtained from such measurements were in tension with other cosmological probes (e.g. CMB). As a result, the attention has moved to use the SZ power spectrum as a method to investigate the astrophysical uncertainties in the thermal structure of the ICM. This path is the topic of Chapter7.
The results from all the above cosmological tests should be consistent and complementary, not only among themselves but also when compared to those from other cosmological probes (see Fig.2.7).
CHAPTER
3
X-ray observations
Since the 1970s, astrophysicists have been studying the Universe in X-ray wavelengths. This has been possible thanks to the technological progress that have allowed the develop- ment of X-ray satellite observatories. Today’s X-ray missions, such as XMM-Newton and Chandra, are so advanced that they can last for a decade or longer. However, the current generation of X-ray observatories is approaching the end of their lifetimes. Therefore, a new generation of X-ray missions is about to be launched, like the eROSITA mission, or are in preparation, like the ATHENA mission. The technical capabilities, such as imaging and spectroscopy, of both missions will be more powerful than their predecessors.
X-ray sources comprise almost all classes of astronomical objects, from planets and stars out to black holes and galaxy clusters. The ROSAT satellite performed the first X-ray all- sky survey in 1990, detecting thousands of X-ray sources, although mostly bright and low redshift objects. With the advent of more sophisticated X-ray missions, the detection of fainter and more distant sources has become possible. At the same time, the amount of X- ray data has enormously increased during the past 25 years through different and dedicated X-ray surveys. Thus, the sensitivity increment and the huge amount of collected data have led to the development of more complex, reliable, and semi-automatic source detection algorithms. This is of especial importance for X-ray galaxy cluster surveys, where the detection and identification of galaxy groups and clusters must be as complete and pure as possible in order to use such objects as cosmological probes. For a comprehensive knowledge of the detected sample, the systematic effects on galaxy cluster detection must be modelled through extensive X-ray simulations.
This chapter reviews the development of X-ray astronomy during the last 45 years. The X-ray telescopes instrumentation is presented, as well as the current and future generation of X-ray missions. A detailed description of different X-ray surveys, and how X-ray sources are detected and identified is provided. Especial attention is given to galaxy cluster surveys. Finally, different X-ray simulators are discussed.
3.1 X-ray astronomy: a brief overview
Observing the Universe in X-ray is impossible from ground-based observatories since Earth’s atmo- sphere is opaque to X-ray radiation. During the first steps of X-ray astronomy (1960s-1970s), strato- spheric balloons and rockets were used to observe the incident X-ray radiation. In this way, for example, the Sun’s corona was observed at such wavelengths, as well as the first galactic X-ray source, the X-ray binary Scorpious X-1 (Giacconi et al. 1962), and the first galaxy clusters Virgo (Byram et al. 1966), Perseus (Fritz et al.1971) and Coma (Meekins et al.1971).
With the advancement and development of astronomical instrumentation, observational techniques, but especially, of space technology, X-ray astronomy has enormously grown. The main characteristic of X-ray observations is the detection and collection of individual photons one by one. Therefore, X-ray telescopes should be able to determine the arrival direction, energy and time of arrival of the photons. The first X-ray detectors were proportional counters and scintillation counters. The payload of the first X-ray satellite, Uhuru, consisted of two sets of proportional counters, which were sensitive in the [2 − 20] keV energy band. The satellite was launched in 1970 by the National Aeronautics and Space Administration (NASA), and one of its main achievements was the discovery and detailed study of pulsating accretion-powered binary X-ray sources.
The next generation of X-ray observatories introduced focusing and imaging X-ray optics, as well as imaging detectors, providing two-dimensional X-ray images. The first imaging X-ray telescope was the Einstein Observatory. It was also a NASA project and it was launched in 1978. Such advancement improved the sensitivity considerably, as well as the resolution of X-ray measurements. The European X-ray Observatory SATellite (EXOSAT), launched in 1983 by the European Space Agency (ESA), in- cluded two imaging telescopes.
The next major step in X-ray astronomy was provided by the ROentgen SATellit (ROSAT) observatory. It was built under international cooperation between Germany, the United Kingdom and the United States. It was launched in 1990 and it was the first imaging telescope to perform an all-sky survey. The ROSAT mission was able to detect ∼ 125, 000 X-ray sources, thanks to its improved telescopes and technology. The mission ended after eight years. The Einstein and ROSAT observatories also used proportional counters. Proportional counters presented high background radiation, i.e. photons from diffuse and unresolved X-ray sources and from charged cosmic rays particles, making the X-ray observations a challenging task.
The proportional counters were replaced by Charge-Coupled Devices (CCDs) as X-ray detectors. The Japanese-American Advanced Satellite for Cosmology and Astrophysics (ASCA) mission was the first X-ray observatory to use such technology.
There have been around 30 X-ray satellite observatories since Uhuru in 1970. Each new mission comes with an improved technology. Thanks to this, current observatories can last for a decade or longer. In general, with the advent of X-ray satellites it was possible to observe and image the emission of extragalactic sources such as the hot gas in galaxy clusters and active galactic nuclei powered by black holes, as well as galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf, neutron star or black hole X-ray binaries.
In the following sections, X-ray telescope characteristics are described, as well as the current and future X-ray observatories. The methodology for conducting X-ray surveys, source detection and identification techniques are also presented. Finally, different X-ray simulations are described.