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

We combine Lyman-break colour selection with ultradeep (greater than or similar to 200 ks) Chandra X-ray imaging over a survey area of similar to 0.35 deg(2) to select high-redshift active galactic nuclei (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 similar to 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 similar to 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 (L-X less than or similar to 10(42.5) erg s(-1)), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.