THE FIELD X-RAY AGN FRACTION TO z=0.7 FROM THE CHANDRA MULTIWAVELENGTH PROJECT AND THE SLOAN DIGITAL SKY SURVEY

We employ the Chandra Multiwavelength Project (ChaMP) and the Sloan Digital Sky Survey (SDSS) to study the fraction of X-ray-active galaxies in the field to z = 0.7. We utilize spectroscopic redshifts from SDSS and ChaMP, as well as photometric redshifts from several SDSS catalogs, to compile a Parent sample of more than 100,000 SDSS galaxies and nearly 1600 Chandra X-ray detections. Detailed ChaMP volume completeness maps allow us to investigate the local fraction of active galactic nuclei (AGNs), defined as those objects having broadband X-ray luminosities L-X(0.5-8 keV) >= 10(42) erg s(-1), as a function of absolute optical magnitude, X-ray luminosity, redshift, mass, and host color/morphological type. In five independent samples complete in redshift and i-band absolute magnitude, we determine the field AGN fraction to be between 0.16% +/- 0.06% (for z <= 0.125 and -18 > M-i > -20) and 3.80% +/- 0.92% (for z <= 0.7 and M-i < -23). We find excellent agreement between our ChaMP/SDSS field AGN fraction and the Chandra cluster AGN fraction, for samples restricted to similar redshift and absolute magnitude ranges: 1.19% +/- 0.11% of ChaMP/SDSS field galaxies with 0.05 < z < 0.31 and absolute R-band magnitude more luminous than M-R < -20 are AGNs. Our results are also broadly consistent with measures of the field AGN fraction in narrow, deep fields, though differences in the optical selection criteria, redshift coverage, and possible cosmic variance between fields introduce larger uncertainties in these comparisons.