  Sensitive Herschel far-infrared observations can break degeneracies that were
inherent to previous studies of star formation in high-z AGN hosts. Combining
PACS 100 and 160um observations of the GOODS-N field with 2Msec Chandra data,
we detect ~20% of X-ray AGN individually at >3sig. The host far-infrared
luminosity of AGN with L2-10~10^43erg/s increases with redshift by an order of
magnitude from z=0 to z~1. In contrast, there is little dependence of
far-infrared luminosity on AGN luminosity, for L2-10<~10^44erg/s AGN at z>~1.
We do not find a dependence of far-infrared luminosity on X-ray obscuring
column, for our sample which is dominated by L2-10<10^44erg/s AGN. In
conjunction with properties of local and luminous high-z AGN, we interpret
these results as reflecting the interplay between two paths of AGN/host
coevolution. A correlation of AGN luminosity and host star formation is traced
locally over a wide range of luminosities and also extends to luminous high z
AGN. This correlation reflects an evolutionary connection, likely via merging.
For lower AGN luminosities, star formation is similar to that in non-active
massive galaxies and shows little dependence on AGN luminosity. The level of
this secular, non-merger driven star formation increasingly dominates over the
correlation at increasing redshift.
