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We study the co-existence of star formation and AGN activity in X-rayselected AGN by analyzing stacked 870um submm emission from a deep and wide mapof the Extended Chandra Deep Field South, obtained with LABOCA at the APEXtelescope. The total X-ray sample of 895 sources with median redshift z~1 isdetected at a mean submm flux of 0.49+-0.04mJy, corresponding to a typical starformation rate around 30Msun/yr for a T=35K, beta=1.5 greybody far-infraredSED. The good S/N permits stacking analyses for subgroups. We observe a trendof star formation rate increasing with redshift. An increase of star formationrate with AGN luminosity is indicated at the highest L_2-10>~1E44erg/sluminosities only. Increasing trends with X-ray obscuration as expected in someAGN evolutionary scenarios are not observed for the bulk of the X-ray AGNsample but may be present for the highest intrinsic luminosity objects. Thissuggests a transition between two modes in the coexistence of AGN activity andstar formation. For the bulk of the sample, the X-ray luminosity andobscuration of the AGN are not intimately linked to the global star formationrate of their hosts. The hosts are likely massive and forming stars secularly,at rates similar to the pervasive star formation seen in massive galaxieswithout an AGN at similar redshifts. The change indicated towards more intensestar formation, and a more pronounced increase in star formation rates betweenunobscured and obscured AGN at highest luminosities suggests that luminous AGNfollow an evolutionary path on which obscured AGN activity and intense starformation are linked, possibly via merging. Comparison to local hard X-rayselected AGN supports this interpretation. [Abridged]

The LABOCA survey of the Extended Chandra Deep Field South: Two modes of star formation in AGN hosts?