Cold dust emission from X-ray AGN in the SCUBA-2 Cosmology Legacy Survey: dependence on luminosity, obscuration and AGN activity

We study the 850-μm emission in X-ray-selected active galactic nuclei (AGN) in the ∼2 deg² COSMOS field using new data from the SCUBA-2 Cosmology Legacy Survey. We find 19 850-μm bright X-ray AGN in a `high-sensitivity' region covering 0.89 deg² with flux densities of S₈₅₀ = 4-10 mJy. The 19 AGN span the full range in redshift and hard X-ray luminosity covered by the sample - 0.7 ≲ z ≲ 3.5 and 43.2 ≲ log₁₀(L_X) ≲ 45. We report a highly significant stacked 850-μm detection of a hard X-ray flux-limited population of 699 z > 1 X-ray AGN - S₈₅₀ = 0.71 ± 0.08 mJy. We explore trends in the stacked 850-μm flux densities with redshift, finding no evolution in the average cold dust emission over the redshift range probed. For type 1 AGN, there is no significant correlation between the stacked 850-μm flux and hard X-ray luminosity. However, in type 2 AGN the stacked submillimeter flux is a factor of 2 higher at high luminosities. When averaging over all X-ray luminosities, no significant differences are found in the stacked submillimeter fluxes of type 1 and type 2 AGN as well as AGN separated on the basis of X-ray hardness ratios and optical-to-infrared colours. However, at log₁₀(L_{2 - 10}/erg s^-1) > 44.4, dependences in average submillimeter flux on the optical-to-infrared colours become more pronounced. We argue that these high-luminosity AGN represent a transition from a secular to a merger-driven evolutionary phase where the star formation rates and accretion luminosities are more tightly coupled. Stacked AGN 850-μm fluxes are compared to the stacked fluxes of a mass-matched sample of K-band-selected non-AGN galaxies. We find that at 10.5 <log₁₀(M_*/M_⊙)<11.5, the non-AGN 850-μm fluxes are 1.5-2 times higher than in type 2 AGN of equivalent mass. We suggest these differences are due to the presence of massive dusty, red starburst galaxies in the K-band-selected non-AGN sample, which are not present in optically selected catalogues covering a smaller area.