A Joint Model of the X-ray And Infrared Extragalactic Backgrounds: I. Model Construction And First Results

Abstract: We present an extragalactic population model of the cosmic background light to interpret the rich high-quality survey data in the X-ray and IR bands. The model incorporates star-formation and supermassive black hole (SMBH) accretion in a co-evolution scenario to fit simultaneously 617 data points of number counts, redshift distributions and local luminosity functions (LFs) with 19 free parameters. The model has four main components, the total IR LF, the SMBH accretion energy fraction in the IR band, the star-formation SED and the unobscured SMBH SED extinguished with a HI column density distribution. As a result of the observational uncertainties about the star-formation and SMBH SEDs, we present several variants of the model. The best-fit reduced chi^2 reaches as small as 2.7-2.9 of which a significant amount (>0.8) is contributed by cosmic variances or caveats associated with data. Compared to previous models, the unique result of this model is to constrain the SMBH energy fraction in the IR band that is found to increase with the IR luminosity but decrease with redshift up to z ~ 1.5; this result is separately verified using aromatic feature equivalent width data. The joint modelling of X-ray and mid-IR data allows for improved constraints on the obscured AGN, especially the Compton-thick AGN population. All variants of the model require that Compton-thick AGN fractions decrease with the SMBH luminosity but increase with redshift while the type-1 AGN fraction has the reverse trend.