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CHAPTER 6: SUMMARY OF THE RESULTS survey was divided in four subsamples in the following energy ranges: soft (0.5-2 keV), hard (2-10 keV), XID (0.5-4.5 keV) and ultrahard (4.5- 7.5 keV), in which we ended up with 1267, 397, 1359 and 91 X-ray sources, respectively. • XMS (XMM-Newton Medium Sensitivity Survey, [12]): a serendipitous X- ray source survey at medium fluxes built from the AXIS survey. It covers a geometric area of ∼3 deg 2 and also consists of four overlapping subsamples in the soft, hard, XID and ultrahard energy bands. The soft, hard and XID subsamples are flux limites while the ultrahard subsample is not artificially limited due to the scarcity of its sources. An extensive identification programme was carried out on the XMS survey, ending up with 272 out of distinct 318 sources spectroscopically identified (86% overall, over 90% in the soft and XID bands). Most of the identified sources turned out to be Type-1 AGN (∼70% in the soft and XID bands, ∼55% in the hard and ultrahard bands) while the rest were identified as Type-2 AGN (∼20%), galaxies (∼5%) and stars (∼4%). These surveys contribute the main samples used in the investigations carried out in this thesis but, in order to gather broader statistics, we have combined them with other complementary XMM-Newton surveys as well as with surveys from other present and past X-ray observatories such as ROSAT, ASCA or Chandra. Shallower, wide area surveys provide brigh sources, while deep pencil-beam surveys probe sources at fainter fluxes. The main aims enumerated in Chapter 1 that we have addressed throughout this thesis are: 1. The Cosmic X-ray Background (CXB) is a record of the accretion power history of the Universe and it is now accepted that it comes from the integrated emission of discrete extragalactic sources. Both absorbed (Type- 2) and unabsorbed (Type-1) AGN are the main contributors to the CXB, with galaxy populations only contributing significantly at fainter X-ray fluxes. We have calculated the sky density or source counts of the AXIS sources, since these distributions are dependent on the cosmological properties that configure the CXB emission. The more accurate the shape of the source counts is constrained, the more precise will be the contribution from discrete sources to the CXB. 148
CHAPTER 6: SUMMARY OF THE RESULTS We have therefore used our best fit logN − logS to calculate both the total resolved fraction of the Cosmic X-ray Background (including the contribution from bright sources) and the relative contribution in different flux bins to the total CXB). We have used the estimates of the average CXB intensity in the Soft and Hard bands from [160] and converted them to the XID and Ultrahard bands using a power law spectrum with Γ = 1.4. The total resolved fraction down to the lowest fluxes of our combined sample reaches 88% in the Soft band and 86% in the Hard band (where we have been able to reach deep fluxes usin pencil beam surveys [19]), but it is only 60% in the XID band and 25% in the Ultrahard band. The total intensity produced extrapolating our logN − logS to zero flux does not saturate the CXB intensity. Assuming that another flux break occurs just below our detected minimum fluxes, we have estimated the minimum slope needed to reproduce the CXB with only discrete sources obtaining values of 1.85 in the Soft band and 1.84 in the Hard band. If we compare these values with those of the galaxy and AGN source counts from the Chandra Deep Field (CDF, [19]), we find that galaxies could easily provide this steepening and, among the AGN population, only the absorbed ones could do so as well. The maximum fractional contribution to the CXB in the Soft, Hard and XID bands comes from sources within a decade around ∼ 10 −14 cgs (where the break in the source counts power law approximately occurs). This is almost 50% of the total contribution in the Soft and Hard bands, which means that medium deep surveys such as AXIS are essential to understand the evolution of the X-ray emission in the Universe up to 10 keV. 2. The dominant population at medium and low X-ray fluxes are AGN, which are known to cluster strongly (i.e. [242], [84], [161], [37]). Since to detect source clustering it is needed a survey that it is both deep (to achieve high angular density) and wide (to prevent a single structure to bias the overall average), the AXIS and XMS surveys are ideal to investigate the large scale structure of AGN. Angular clustering studies the excess probability of finding a pair of sources at a given angular distance projected in the sky. We have studied it in two ways. The first is the standard two-point angular correlation function w(θ). Using this method we have detected evidence of clustering at about the 99% level in the Soft and XID bands but not in the Hard and Ultrahard 149
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CONTENTS 1.6.1 The K-correction . .
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CONTENTS 5.3.2 Analytical model . .
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LIST OF FIGURES 4.6 Soft 1 − P µ
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Chapter 1 Introduction In the last
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