Figure 20: Expected ratio of the observed-frame hard-to-soft flux as a function of obscuring neutral Hydrogen column density ( ). The black solid line was derived assuming an intrinsic power law spectrum with slope and photoelectric absorption. The gray zone shows the measured ratio for the stack of galaxies at and the standard deviation limits. A column density of  cm−2, that is, Compton-thick obscuration, is required to explain the observed hard-to-soft X-ray flux ratio [152].