The evolving X-ray spectrum of active galactic nuclei: evidence for an increasing reflection fraction with redshift
Abstract
The cosmic X-ray background (XRB) spectrum and active galaxy number counts encode essential information about the spectral evolution of active galactic nuclei (AGNs) and have been successfully described by XRB synthesis models for many years. Recent measurements of the 8-24 keV AGN number counts by NuSTAR and Swift-BAT are unable to be simultaneously fit by existing XRB synthesis models, indicating a fundamental breakdown in our understanding of AGN evolution. Here we show that the hard X-ray number counts can be successfully modeled with an XRB synthesis model in which the reflection strength in the spectral model increases with redshift. We show that a similar increase in reflection strength is a natural outcome of connecting the reflection to the incidence of high column density gas and the growth in the AGN obscured fraction to higher redshifts. However, this physically-motivated model of the evolution of the reflection strength cannot entirely account for the NuSTAR number counts. Therefore, an additional hard X-ray dominated population of AGNs -- evolving separately from the main population -- must be included in future XRB synthesis models. We suggest that Compton-thick AGNs associated with major galaxy mergers may be the missing component.
- Publication:
-
American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23324324A