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

Stanley, F; Harrison, CM; Alexander, DM; Simpson, J; Knudsen, KK; Mullaney, J R; Rosario, DJ; Scholtz, J

Abstract

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.

Citation

Stanley, F., Harrison, C., Alexander, D., Simpson, J., Knudsen, K., Mullaney, J., Rosario, D., & 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. https://doi.org/10.1093/mnras/sty1044

• Journal Article Type: Article
• Acceptance Date: Apr 23, 2018
• Online Publication Date: May 7, 2018
• Publication Date: Aug 21, 2018
• Deposit Date: Sep 5, 2018
• Publicly Available Date: Sep 18, 2018
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 478
• Issue: 3
• Pages: 3721-3739
• DOI: https://doi.org/10.1093/mnras/sty1044
• Public URL: https://durham-repository.worktribe.com/output/1315777