  Observations in the last decade have provided strong evidence that the growth
of supermassive black holes at the centres of galaxies is among the most
influential processes in galaxy evolution. Open questions that relate to our
current understanding of black hole growth and its relation to the build-up of
galaxies at redshifts z=1-4, when most black holes and stars we see in
present-day galaxies were put in place, include: what is the nature of AGN
feedback and whether it plays a significant role in the evolution of galaxies?
what is the dominant population of accreting AGN at that critical epoch? is it
dominated by obscured objects as required by many current observations and
models? The Athena+ mission concept will provide the technological leap
required for a breakthrough in our understanding of AGN and galaxy evolution at
the heyday of the Universe. The high throughput of Athena+ will allow the
systematic study of the incidence, nature and energetics of AGN feedback
processes to z~4 via the identification and measurement of blue-shifted X-ray
absorption lines. The excellent survey and spectral capabilities of the Athena+
Wide Field Imager will complete the census of black hole growth by yielding
samples of up to 100 times larger than is currently possible of the most
heavily obscured, including Compton thick, AGN to redshifts z~4. The
demographics of this population relative to their hosts is fundamental for
understanding how major black hole growth events relate to the build-up of
galaxies.
