Deep ALMA photometry of distant X-ray AGN: improvements in star formation rate constraints, and AGN identification

Abstract

We present the star formation rates (SFRs) of a sample of 109 galaxies with X-ray-selected active galactic nuclei (AGNs) with moderate to high X-ray luminosities (L_2-8 keV = 10^{42}-10^{45} erg s^{-1}), at redshifts 1 < z < 4.7, that were selected to be faint or undetected in the Herschel bands. We combine our deep Atacama large (sub-)millimetre array (ALMA) continuum observations with deblended 8-500 μ m photometry from Spitzer and Herschel, and use infrared (IR) spectral energy distribution (SED) fitting and AGN - star formation decomposition methods. The addition of the ALMA photometry results in an order of magnitude more X-ray AGN in our sample with a measured SFR (now 37 %). The remaining 63 % of the sources have SFR upper limits that are typically a factor of 2-10 times lower than the pre-ALMA constraints. With the improved constraints on the IR SEDs, we can now identify a mid-IR (MIR) AGN component in 50 % of our sample, compared to only ∼1 % previously. We further explore the F_{870 μ m}/F_{24 μ m}-redshift plane as a tool for the identification of MIR-emitting AGN, for three different samples representing AGN-dominated, star formation-dominated, and composite sources. We demonstrate that the F_{870 μ m}/F_{24 μ m}-redshift plane can successfully split between AGN and star formation-dominated sources, and can be used as an AGN identification method.