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

TL;DR

This study measures the fluctuations in the cosmic X-ray background using Chandra data, revealing that unresolved galaxy clusters and groups significantly contribute to large-scale surface brightness variations, providing insights into their distribution and properties.

Contribution

First measurement of CXB fluctuation power spectrum on arcsecond to arcminute scales, demonstrating the dominant role of unresolved galaxy clusters and groups at larger scales.

Findings

Sub-arcmin fluctuations consistent with AGN shot noise.

Detection of large-scale LSS signal from unresolved clusters.

Clusters contribute significantly to CXB fluctuations at large scales.

Abstract

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 Chandra data of the XBOOTES field ($\sim9\,\mathrm{deg^2}$) to conduct the most accurate measurement to date of the power spectrum of fluctuations of the unresolved CXB on the angular scales of $\sim3\,$arcsec $-$ $\sim17\,$arcmin. We find that at sub-arcmin angular scales, the power spectrum is consistent with the AGN shot noise, without much need for any significant contribution from their one-halo term. This is consistent with the theoretical expectation that low-luminosity AGN reside alone in their dark matter halos. 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 $-0.8\pm0.1$ 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 $\left<z\right>\sim0.3$, and the mean temperature of their intracluster medium (ICM), $\left<T\right>\approx 1.4$ keV, corresponds to the mass of $M_{500} \sim 10^{13.5}\,\mathrm{M}_\odot$. 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 $\sim$Mpc, i.e. of the order of the virial radius.