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
out 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 1,600 Chandra X-ray detections.
Detailed ChaMP volume completeness maps allow us to investigate the local
fraction of active galactic nuclei (AGN), defined as those objects having
broad-band 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 striking 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 AGN. 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.
