A remarkably flat relationship between the average star formation rate and AGN luminosity for distant X-ray AGN

Stanley, F.; Harrison, C.M.; Alexander, D.M.; Swinbank, A.M.; Aird, J.A.; Del Moro, A.; Hickox, R.C.; Mullaney, J.R.

Abstract

In this study, we investigate the relationship between the star formation rate (SFR) and AGN luminosity (LAGNLAGN) for ∼2000 X-ray detected AGN. The AGN span over three orders of magnitude in X-ray luminosity (1042<L2−8keV<1045.5ergs−11042<L2−8keV<1045.5ergs−1) and are in the redshift range z = 0.2–2.5. Using infrared (IR) photometry (8–500 μmμm), including deblended Spitzer and Herschel images and taking into account photometric upper limits, we decompose the IR spectral energy distributions into AGN and star formation components. Using the IR luminosities due to star formation, we investigate the average SFRs as a function of redshift and AGN luminosity. In agreement with previous studies, we find a strong evolution of the average SFR with redshift, tracking the observed evolution of the overall star-forming galaxy population. However, we find that the relationship between the average SFR and AGN luminosity is broadly flat at all redshifts and across all the AGN luminosities investigated; in comparison to previous studies, we find less scatter amongst the average SFRs across the wide range of AGN luminosities investigated. By comparing to empirical models, we argue that the observed flat relationship is due to short time-scale variations in AGN luminosity, driven by changes in the mass accretion rate, which wash out any underlying correlations between SFR and LAGNLAGN. Furthermore, we show that the exact form of the predicted relationship between SFR and AGN luminosity (and its normalization) is highly sensitive to the assumed intrinsic Eddington ratio distribution.

Citation

Stanley, F., Harrison, C., Alexander, D., Swinbank, A., Aird, J., Del Moro, A., …Mullaney, J. (2015). A remarkably flat relationship between the average star formation rate and AGN luminosity for distant X-ray AGN. Monthly Notices of the Royal Astronomical Society, 453(1), 591-604. https://doi.org/10.1093/mnras/stv1678

• Journal Article Type: Article
• Acceptance Date: Jul 22, 2015
• Online Publication Date: Aug 17, 2015
• Publication Date: Oct 11, 2015
• Deposit Date: Feb 9, 2016
• Publicly Available Date: Feb 12, 2016
• 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: 453
• Issue: 1
• Pages: 591-604
• DOI: https://doi.org/10.1093/mnras/stv1678
• Keywords: Galaxies: active, Ggalaxies: evolution, Infrared: galaxies, X-rays: galaxies.
• Public URL: https://durham-repository.worktribe.com/output/1412760