ar X iv : a st ro - p h / 05 09 35 6 v 3 2 6 Ja n 20 06 Connecting Galaxy Evolution , Star Formation and the X - ray Background

As a result of deep hard X-ray observations by Chandra and XMM-Newton a significant fraction of the cosmic X-ray background (CXRB) has been resolved into individual sources. These objects are almost all active galactic nuclei (AGN) and optical followup observations find that they are mostly obscured Type 2 AGN, have Seyfert-like X-ray luminosities (i.e., L X ∼ 10 43−44 ergs s −1), and peak in redshift at z ∼ 0.7. Since this redshift is similar to the peak in the cosmic star-formation rate, this paper proposes that the obscuring material required for AGN unification is regulated by star-formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of Type 2/Type 1 AGN that is a function of both z and 2–10 keV X-ray luminosity, L X. The evolutionary models are constrained by parameterizing the observed Type 1 AGN fractions from the recent work by Barger et al. The parameterization which simultaneously best accounts for Barger's data, the CXRB spectrum and the X-ray number counts has a local, low-L X Type 2/Type 1 ratio of 4, and predicts a Type 2 AGN fraction which evolves as (1 + z) 0.3. Models with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1–2 for log L X > 44, and thus the deep X-ray surveys are missing about half the obscured AGN with these luminosities. These objects are likely to be Compton thick. The parameterization also predicts a covering fraction approaching unity at z ∼ 1 and log L X ∼ 41.5 which would be very difficult to explain using a simple unevolving torus model. Overall, these calculations show that the current data strongly supports a change to the AGN unification scenario where the obscuration is connected with star formation in the – 2 – host galaxy rather than a molecular torus alone. The evolution of the obscuration implies a close relationship between star formation and AGN fueling, most likely due to minor mergers or interactions.