We study the co-existence of star formation and AGN activity in Chandra X-ray selected AGN by analyzing stacked 870 \mu m submm emission from a deep and wide map of the Extended Chandra Deep Field South , obtained with the LABOCA instrument at the APEX telescope .
The total X-ray sample of 895 sources with median redshift z \sim 1 drawn from the combined ( E ) CDFS X-ray catalogs is detected at > 11 \sigma significance at a mean submm flux of 0.49 \pm 0.04 mJy , corresponding to a typical star formation rate around 30 M _ { \odot } yr ^ { -1 } for a T=35K , \beta =1.5 greybody far-infrared spectral energy distribution .
The good signal to noise ratio permits stacking analyses for major subgroups , splitting the sample by redshift , intrinsic luminosity , and AGN obscuration properties .
We observe a trend of star formation rate increasing with redshift .
An increase of star formation rate with AGN luminosity is indicated at the highest L _ { 2 - 10 keV } \gtrsim 10 ^ { 44 } { erg s } ^ { -1 } luminosities only .
Increasing trends with X-ray obscuration as expected in some AGN evolutionary scenarios are not observed for the bulk of the X-ray AGN sample but may be present for the highest intrinsic luminosity objects with L _ { 2 - 10 keV } \gtrsim 10 ^ { 44 } { erg s } ^ { -1 } .
This behaviour suggests a transition between two modes in the coexistence of AGN activity and star formation .
For the bulk of the sample , the X-ray luminosity and obscuration of the AGN are not intimately linked to the global star formation rate of their hosts .
The hosts are likely massive and forming stars secularly , at rates similar to the pervasive star formation seen in massive galaxies without an AGN at similar redshifts .
In these systems , star formation is not linked to a specific state of the AGN and the period of moderately luminous AGN activity may not highlight a major evolutionary transition of the galaxy .
The change indicated towards more intense star formation , and a more pronounced increase in star formation rates between unobscured and obscured AGN reported in the literature at highest ( L _ { 2 - 10 keV } \gtrsim 10 ^ { 44 } { erg s } ^ { -1 } ) luminosities suggests that these luminous AGN follow an evolutionary path on which obscured AGN activity and intense star formation are linked , possibly via merging .
Comparison to local hard X-ray selected AGN supports this interpretation .
Star formation rates in the hosts of moderate luminosity AGN at z \sim 1 are an order of magnitude higher than at z \sim 0 , following the increase in the non-AGN massive galaxy population .
At high AGN luminosities , hosts on the evolutionary link/merger path emerge from this secular level of star formation .