The X-ray luminosity function of AGN at z~3

TL;DR

This study uses deep X-ray imaging and optical selection to accurately determine the faint end of the AGN X-ray luminosity function at z~3, revealing no flattening of the faint slope and significant negative evolution of low luminosity AGN.

Contribution

It presents the most accurate measurement to date of the faint end of the high-redshift AGN XLF using combined optical and X-ray data with careful corrections.

Findings

No flattening of the faint slope at z~3 compared to z=1.

Significant negative evolution (~3.6 times decrease) of low luminosity AGN space density.

Upturn in the XLF at very low luminosities likely due to starburst galaxies.

Abstract

We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN. Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z=1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ~3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.