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Kolodzig, Alexander (2015): Large-scale structure studies using AGN in X-ray surveys: Challenges from XBOOTES and prospects for eROSITA. Dissertation, LMU München: Fakultät für Physik
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Abstract

Large X-ray surveys are a powerful tool to study the large-scale structure (LSS) of the Universe. The scientific impact of LSS studies using active galactic nuclei (AGN) in X-ray surveys can be significantly increased by conducting wider and deeper X-ray surveys and studying the surface brightness fluctuations of the unresolved cosmic X-ray background (CXB). In the first part of this Thesis, we have investigated the prospects of using the AGN sample to be detected by the upcoming eROSITA all-sky survey (eRASS) for LSS studies. We show that eRASS will detect about 3 million AGN in the 0.5-2.0 keV band. This will result in a ~30 times larger number of sources and a ~30 times better sensitivity than its 25 year old predecessor, the ROSAT all-sky survey (RASS). We show that this unprecedented AGN sample will have a median luminosity of ~10^44 erg/s, which is typical for the entire AGN population in this energy band. It will have a median redshift of z ~ 1 and approximately 40% of the objects will be in the redshift range of z = 1-2, where the bulk of the X-ray emission of AGN is produced. About 10^4 - 10^5 AGN are predicted to be beyond redshift z = 3 and about 2 000 - 30 000 beyond z = 4, which will potentially include some of the earliest AGN in the Universe. We demonstrate that, given these unique properties, the eRASS-AGN sample will be able to significantly improve our current knowledge of the AGN spatial density as a function of redshift and luminosity over a wide range of cosmic time. Further, we show that it will enable us, for the first time, to perform detailed redshift- and luminosity-resolved studies of the clustering strength of X-ray selected AGN. All these measurements will dramatically improve our understanding of the growth of supermassive black holes over cosmic time and its implications for galaxy evolution. We demonstrate for the first time that, given the breadth and depth of eRASS, it will be possible to use AGN as a cosmological probe via baryon acoustic oscillation (BAO) measurements. We will be able to convincingly detect BAOs in the currently uncharted redshift range of z ~ 1-2, which will improve the constraints on the current cosmological model. In the second part of this Thesis, we have conducted the most accurate measurement to date of the brightness fluctuations of the unresolved CXB in the 0.5-2.0 keV band for angular scales of < ~17'. For this we used the XBOOTES survey, the currently largest continuous survey of the X-ray telescope Chandra. We find that on small angular scales (< ~2') the observed power spectrum of the brightness fluctuations is broadly consistent with the conventional AGN clustering model, although with a 30% deviation. This deviation nevertheless presents a good opportunity to improve our understanding of clustering properties of unresolved AGN by testing more sophisticated clustering models with our measurement. For angular scales of > ~2' we measure a significant excess with up to an order of magnitude difference in comparison to the standard AGN clustering model. We demonstrate that an instrumental origin can be excluded. However, we also show that the excess can neither be explained with any known X-ray source population by looking at strength of its clustering signal and the shape of its energy spectrum. It might be caused by more than one type of source but the dominant source appears to have extragalactic origin. Finally, we make predictions on how eRASS will be able to advance the studies of the unresolved CXB.

Abstract