Constraining the AGN Luminosity Function from JWST with the X-Ray Background

Abstract

We predict the X-ray background (XRB) expected from the population of quasars detected by the James Webb Space Telescope spectroscopic surveys over the redshift range z ∼ 4–7. We find that the measured UV emissivities imply a ∼10 times higher unresolved XRB than constrained by current experiments. We illustrate the difficulty of simultaneously matching the faint end of the quasar luminosity function and the XRB constraints. We discuss possible origins and consequences of this discrepancy.

Cosmic Reionization in the JWST Era: Back to AGNs?

“…Constraints from the X-Ray Background Despite nondetections in the X-ray, as may be expected if they are below current survey limits, the population of broadline AGNs observed in recent JWST surveys may contribute to the observed X-ray background (XRB). Indeed, it has been recently argued by Padmanabhan & Loeb (2023) that the inferred AGN UV emissivities at z > 5 imply an unresolved XRB that is about 1 order of magnitude higher than constrained by current experiments. Here, we show that this is actually not the case.…”

“…Our results may therefore be slightly biased compared to radiation-hydrodynamic simulations(Chen et al 2020). 14 We note that Equation (3) inPadmanabhan & Loeb (2023), defining the "Olbers' integral" for the specific intensity of the XRB is actually incorrect. In their integrand expression there is a missing factor of (1 + z) in the numerator associated with the reduction of the bandwidth.…”