In 2014, the Advanced Telescope for High Energy Astrophysics (ATHENA) was selected by ESA as the next generation of L-class mission within the Cosmic Vision programme. This X-ray observatory will study the the hot and energetic processes in the Universe. ATHENA aims to answer to fundamental questions: i) How do black holes grow and shape the Universe?, and ii) How does ordinary matter assemble into the large-scale structures that we see today?.
By the time ATHENA is launched in 2028, it is expected that the parameters of our cosmological model will be tightly constrained by other missions, such as eROSITA and Euclid. However, major astrophys- ical questions related to the formation and evolution of the largest collapsed structures, namely galaxy groups and clusters, will remain. A large X-ray observatory like ATHENA will allow observing and studying the earliest galaxy groups (z > 2) thanks to its combination of high sensitivity, large effective area, and good spatial and spectral resolution. This will contribute to our understanding of how and when the first galaxy groups in the Universe were formed.
5.5 Summary and conclusions
In this work, the ATHENA capabilities on the detection of high-redshift galaxy groups were evaluated through extensive and dedicated image simulations. These simulations take into account the main in- strumental features of ATHENA: X-ray and particle background, vignetting and PSF degradation with off-axis angle. Galaxy groups are simulated with realistic surface brightness profiles. The simulations also contain a realistic AGN population. The source detection process was done by a wavelet-based al- gorithm combined with the SExtractor software, and the source classification was performed through a maximum likelihood fitting.
The main results showed that high-redshift galaxy groups with masses of M500 = 5 × 1013M at z > 1 will be detected with high probability (> 80%) as extended sources by ATHENA in 100 and 30 ks simulated observations.
Since the hot gas properties in galaxy groups at redshift larger than 2 are unknown, several models were tested. These models are physically motivated from known local galaxy group properties. For example, central AGN contamination, different X-ray luminosity evolution models, and distinct surface brightness profiles. The general outcome of the simulations is that ATHENA will help to constrain structure formation and evolution models as well as feedback models by the number of detected galaxy groups at high redshift.
Distinct performance parameters for the ATHENA instrumental setup were also tested in order to define the science requirements for finding the earliest galaxy groups. The examined performance parameter were different effective area, PSF degradation with off-axis angle, spatial resolution, and instrumental background. The results show that galaxy groups at high redshift can be detected as extended sources by ATHENA when the key instrumental parameters are a large effective area (∼ 2.1 m2at 1 keV) and a good spatial resolution (< 1000) over the full FoV.
The capacity of ATHENA in detecting high-redshift groups is promising due to its high sensitivity and good angular resolution. It is worth to stress that state-of-the-art source detection and characterisation algorithms also play an important role in the detection. From the obtained results, one can see that wavelet-based detection algorithms are efficient in detecting faint extended sources, and that maximum- likelihood fitting algorithms are key for source classification.
CHAPTER
6
Study of faint X-ray sources
Progress in our understanding of galaxy clusters and their use as precision cosmological probes requires a deeper multi-wavelength analysis of clusters. Galaxy clusters are identi- fied by various observational techniques. These methods are sensitive to distinct physical components of the galaxy clusters. For example, X-ray techniques are sensitive to the ex- tended bremsstrahlung emission arising from the hot intra-cluster medium, while optical and infrared observations identify the light coming from the cluster galaxy members. Then, multi-wavelength observations of galaxy clusters are necessary because they allow to un- derstand the different physical states occupied by galaxy clusters at any cosmic epoch. This is important since the ultimate goal is to obtain an accurate census of clusters which leads towards a more accurate constraints on our cosmological model.
In this chapter, a comparison of two samples of high-redshift (z > 0.8) galaxy clusters, selected in the mid-infrared and X-ray bands, is presented. The aim is to study the physical differences between these galaxy cluster samples. Since some mid-infrared selected clusters are not detected in the X-ray bands, new and sophisticated tools are developed to study them.
The two techniques to study faint X-ray sources are: a Bayesian aperture photometry method and a stacking procedure. The former method uses Bayesian inference and takes into account the Poisson nature of the X-ray data. This allows to extract as much in- formation as possible for a given source. The second technique allows to study the mean properties of a given sample by stacking their X-ray data. Both methods are developed in the framework of data obtained by the XMM-Newton observatory.
Note: Section6.4of this chapter is adapted from a paper of the same title. This manuscript will be submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) Journal. Since I am a co-author of the paper, I have adapted the manuscript for this thesis. The first two sections of this chapter correspond to Sections 4.1 and 5.5 of such manuscript and describe in detail the methods I have developed for the study of faint X-ray sources. The reference is Willis, J., Ramos-Ceja, M. E., Pacud, F., and Muzzin, A., 2016, MNRAS, to be submitted.
6.1 Studying faint X-ray sources
Current X-ray missions have been able to detect faint sources due to their high sensitivity and long exposure observations. Such sources are of great interest since they have never been observed before. Given the low flux of such sources, it is important to develop careful methods that allow extracting as much information as possible from the available data. For instance, such methods must take into account the Poisson nature of X-ray data, i.e. the low number counts of photons in X-ray images, in order to obtain useful quantities that describe faint X-ray sources.
In this chapter, two different techniques for studying faint X-ray sources are described. First, a Bayesian aperture photometry method is presented. This approach takes into account the Poisson aspect of the X- ray data and models the background noise. Second, a stacking method is described. It basically consists in adding together X-ray image cut-outs of sources. The aim is to enhance the information and to study the collective properties of a given sample.
In the final section of this chapter, an application of the above methods is presented. The techniques are applied to a mid-infrared selected sample of high redshift (z > 0.8) galaxy clusters. Most of these objects are not individually detected in X-rays. Hence, the necessity of using the Bayesian aperture photometry and stacking methods to investigate their X-ray emission. The aim of this study is to understand how the physical properties of galaxy cluster samples differ depending upon the observational technique used to identify them.