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A.C. Fabian (Institute of Astronomy, Cambridge, UK)
The hard spectrum of the X-ray Background indicates that most of its component sources are absorbed, which in turn means that most accretion - basically that which has led to the massive black hole mass density in the Universe - is obscured. The argument behind this, and its uncertainties, will be explored and a model for the buildup of black holes, in which the properties of both the X-ray Background and local black hole masses are obtained, will be presented. Feedback in the form of a wind from the accreting sources to the surrounding medium is required. The energy and momentum thereby injected can have important consequences for the continued formation of the host galaxy.