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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Stanley, F, Harrison, C M, Alexander, D M, Simpson, J, Knudsen, K K, Mullaney, J R, Rosario, D J & Scholtz, J (2018). Deep ALMA photometry of distant X-ray AGN: improvements in star formation rate constraints, and AGN identification. Monthly Notices of the Royal Astronomical Society 478(3): 3721-3739.

Author(s) from Durham


We present the star formation rates (SFRs) of a sample of 109 galaxies with X-rayselected
active galactic nuclei (AGNs) with moderate to high X-ray luminosities (L2−8 keV
= 1042 − 1045 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 μm 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 F870 μm/F24 μm–redshift plane as a tool for the identification of
MIR-emitting AGN, for three different samples representing AGN-dominated, star formationdominated,
and composite sources. We demonstrate that the F870 μm/F24 μm–redshift plane can
successfully split between AGN and star formation-dominated sources, and can be used as an
AGN identification method.