The Cosmic History of Black Hole Growth from Deep Multiwavelength Surveys

Figure 20

Expected ratio of the observed-frame hard-to-soft flux as a function of obscuring neutral Hydrogen column density (𝑁_𝐻). The black solid line was derived assuming an intrinsic power law spectrum with slope Γ=1.9 and photoelectric absorption. The gray zone shows the measured ratio for the stack of galaxies at 𝑧≃6 and the ±1 standard deviation limits. A column density of 𝑁_𝐻≃10²⁴ cm⁻², that is, Compton-thick obscuration, is required to explain the observed hard-to-soft X-ray flux ratio [152].