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

Journal article, 2018

We present the star formation rates (SFRs) of a sample of 109 galaxies with
X-ray selected active galactic nuclei (AGN) with moderate to high X-ray luminosities (L(2-8keV) = 10^42 - 10^45 erg/s),
at redshifts 1 < z <4.7, that were selected to be faint or undetected in the Herschel bands.
We combine our deep ALMA continuum observations with deblended 8-500um photometry
from Spitzer and Herschel, and use infrared (IR) 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(870um)/F(24um)--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(870um)/F(24um)--redshift plane can successfully split between AGN and star formation
dominated sources, and can be used as an AGN identification method.