accretion history 97
Figure 4.5: Reconstructed core-excised luminosity as a function of input core-excised luminosity (left panel), as well as reconstructed mass as a function of input mass (right panel). The number of reconstructed photon counts is shown as a color code. The solid lines and shaded regions represent the best-fit linear relation and their intrinsic scatter. The dashed line shows the one-to-one relation. Figures taken fromEckert et al.(2020).
majority of the expected number of detected objects and one is interested in well-calibrated and well-understood scaling relations. Due to their shallower potential well, galaxy groups are more affected by non-gravitational processes than galaxy clusters. In addition, line emission at galaxy group masses becomes significant on top of the bremsstrahlung continuum. Depending on the metallicity, more than 50% of the total X-ray emission can be contributed by line emission. This line emission is not included in the self-similar model. Therefore, galaxy groups seem more luminous and hotter for their mass relative to clusters in scaling relations. This leads, for ex-ample, to a steepening of the slope of the mass-luminosity scaling relation toward low masses (Kettula et al.,2015). This observed mass dependence of scaling relations, together with the fact that the self-similar model neglects the line emission component completely, implies that more complicated scaling-relation models need to be investigated. However, there are no other suit-able functional forms from the theoretical point of view and existing galaxy-cluster samples are too small to model more complicated scaling relations. This will change with the eROSITA all-sky survey, which provides enough statistics to explore scaling-relation models beyond a single power-law. Therefore, it would be valuable to intend to constrain galaxy group scaling relations and quantify the influence of the non-gravitational physics in a future project.
4.3 Probing the link between the scatter of galaxy cluster char-acteristics and the mass accretion history
In addition to study the scaling relations of galaxy groups in the eROSITA all-sky survey, it is important to understand the scatter of the galaxy group mass proxies better. This helps to interpret potential systematic effects in cosmological studies. The ellipticity of the X-ray emission and the
concentration of the dark matter are shaped by the mass accretion history and we plan to explore the correlation between them and the scatter in the core-excised X-ray luminosity. This includes cluster physics in scaling relations and the formalism can easily be extended to additional galaxy group properties, for example optical richness. Additional work is needed to develop combined group mass proxies and to understand the completeness of group selection using X-rays.
Chapter 5
Brief summary
The primary goal of the work presented in this thesis is to outline how to use the X-ray emission from galaxy cluster outskirts for cosmological studies. In Chapter 2, we start by characterizing the X-ray emission of a well defined galaxy cluster sample using a novel approach. The method models the excess emission in cool-core clusters using wavelet decomposition, which provides a better fit to the cluster outskirts. This model provides an unbiased measurement of the extent parameter and the cluster flux in the outskirts with respect to the different core types. In contrast, a widely used classicalβ-model tends to underestimate the flux in the cluster outskirts up to over 40% and the extent parameters are biased low for cool-core clusters. Additionally, we study the scaling of the shape parameters with temperature and the covariances between shape parameters and luminosity at a fixed temperature. For the same mass, more compact objects tend to be more luminous. In Chapter 3 we utilise the findings of Chapter 2 and propose a scheme to detect galaxy clusters with eROSITA through cluster outskirts. Avoiding the cluster centers in source detection minimizes the bias toward cool-core clusters. We determine the detection efficiency of this method by simulations and find that one requires more and more counts with decreasing extent of the cluster to obtain a specific detection efficiency. The method requires better photon statistics for compact clusters to successfully classify the object as extended. The payoffs of this approach are cluster catalogs that are well characterized and have simple selection functions.
Compared to the current eROSITA detection and characterization method, the obtained catalogs are purer at a similar level of completeness. After successful detection in the eROSITA all-sky survey, the galaxy groups and clusters need to be characterized. A self-consistent way to mea-sure the core-excised luminosity will be part of a future study and is presented in Chapter 4.
This method makes use of the self-similar shape of galaxy clusters in the outskirts and predicts the core-excised luminosity and the total mass based on an aperture measurement of the clus-ter flux. First idealized tests on simulations show a scatclus-ter of approximately 36%, which likely sets an upper performance bound of this method when PSF effects are not taken into account.
We present a similar method based on the linear decomposition of observed profiles in Eckert et al.(2020). Applying a proper PSF-reconstruction technique reduces the intrinsic scatter of the core-excised luminosity and mass to approximately 16% without measurable bias. The inferred galaxy cluster parameters are beneficial for cosmological studies. On the one hand, the distribu-tion of the reconstructed total masses can be compared to theoretical predicdistribu-tions to directly infer
cosmological parameters. On the other hand, the reconstructed core-excised luminosities can be utilized in scaling relations. The expected large amount of detected galaxy groups and clusters with eROSITA allows us to study scaling relations in unprecedented detail. This includes study-ing more complex functional forms and the scatter between the different mass proxies. This will help to understand potential systematic effects in cosmological constraints.