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A sample of 198 soft X--ray selected active galactic nuclei (AGN) from theROSAT International X--ray Optical Survey (RIXOS), is used to investigate theX--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2of sky, and is almost completely identified; it is used in combination with theEinstein Extended Medium Sensitivity Survey (EMSS), to give a total sample ofover 600 AGN. We find the evolution of AGN with redshift to be consistent withpure luminosity evolution (PLE) models in which the rate of evolution slowsmarkedly or stops at high redshifts z>1.8. We find that this result is notaffected by the inclusion, or exclusion, of narrow emission line galaxies atlow redshift in the RIXOS and EMSS samples, and is insensitive to uncertaintiesin the conversion between flux values measured with ROSAT and Einstein. Weconfirm, using a model independent Ve/Va test, that our survey is consistentwith no evolution at high redshifts.Comment: 10 pages, LaTeX file, PS figures and mn.sty. Accepted in MNRA

The luminosity function evolution of soft X-ray-selected active galactic nuclei in the RIXOS survey