The star formation rates of active galactic nuclei host galaxies

Using artificial neural network predictions of total infrared luminosities (L_IR), we compare the host galaxy star formation rates (SFRs) of ~21 000 optically selected active galactic nuclei (AGN), 466 low-excitation radio galaxies (LERGs) and 721 mid-IR-selected AGN. SFR offsets (ΔSFR) relative to a sample of star-forming 'main-sequence' galaxies (matched in M*, z and local environment) are computed for the AGN hosts. Optically selected AGN exhibit a wide range of ΔSFR, with a distribution skewed to low SFRs and a median ΔSFR=-0.06 dex. The LERGs have SFRs that are shifted to even lower values with a median ΔSFR=-0.5 dex. In contrast, mid-IR-selected AGN have, on average, SFRs enhanced by a factor of ~1.5. We interpret the different distributions of ΔSFR amongst the different AGN classes in the context of the relative contribution of triggering by galaxy mergers. Whereas the LERGs are predominantly fuelled through low accretion rate secular processes which are not accompanied by enhancements in SFR, mergers, which can simultaneously boost SFRs, most frequently lead to powerful, obscured AGN.