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

Stanley, F. and Harrison, C.M. and Alexander, D.M. and Simpson, J. and Knudsen, K.K. and Mullaney, J.R. and Rosario, D.J. and 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). pp. 3721-3739.


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.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:23 April 2018
Date deposited:18 September 2018
Date of first online publication:07 May 2018
Date first made open access:18 September 2018

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