Can AGN and galaxy clusters explain the surface brightness fluctuations of the cosmic X-ray background?

Fluctuations of the surface brightness of cosmic X-ray background (CXB) carry unique information about faint and low-luminosity source populations, which is inaccessible for conventional large-scale structure (LSS) studies based on resolved sources. We used XBOOTES (5ks deep Chandra X-rayObservatoryACIS-I maps of the similar to 9 deg(2) Bootes field of the NOAO Deep Wide-Field Survey) to conduct the most accurate measurement to date of the power spectrum of fluctuations of the unresolved CXB on the angular scales of 3 arcsec-17 arcmin. We find that at sub-arcmin angular scales, the power spectrum is consistent with the active galactic nucleus ( AGN) shot noise, without much need for any significant contribution from their onehalo term. This is consistent with the theoretical expectation that low-luminosity AGN reside alone in their dark matter haloes. However, at larger angular scales, we detect a significant LSS signal above the AGN shot noise. Its power spectrum, obtained after subtracting the AGN shot noise, follows a power law with the slope of -0.8 +/- 0.1 and its amplitude is much larger than what can be plausibly explained by the two-halo term of AGN. We demonstrate that the detected LSS signal is produced by unresolved clusters and groups of galaxies. For the flux limit of the XBOOTES survey, their flux-weighted mean redshift equals z similar to 0.3, and the mean temperature of their intracluster medium (ICM), T approximate to 1.4 keV, corresponds to the mass of M500 similar to 10(13.5) M-circle dot. The power spectrum of CXB fluctuations carries information about the redshift distribution of these objects and the spatial structure of their ICM on the linear scales of up to similar to Mpc, i. e. of the order of the virial radius.International Max Planck Research School (IMPRS) on Astrophysics at the Ludwig-Maximilians University of Munich (LMU); China Postdoctoral Science Foundation [2016M590012]; Russian Scientific Foundation (RNF) [14-22-00271]SCI(E)ARTICLE33035-305946