| I 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. I 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 an investigation of the local fraction of active galactic nuclei (AGN), defined as those objects having broad-band X-ray luminosities L X(0.5-8 keV) ≥ 1042 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, I determine the field AGN fraction to be between 0.16+/-0.06% (for z ≤ 0.125 and --18 > Mi > --20) and 3.80+/-0.92% (for z ≤ 0.7 and Mi < --23). I find striking agreement between the 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 masses similar to the Milky Way's bulge (∼ 2 x 1010 M⊙ ; equivalent to MR < --20) are AGN. These results are also broadly consistent with measures of the field AGN fraction in narrow, deep fields. I find tentative evidence that the AGN fraction evolves with redshift, consistent with either (1 + z) 3 or (1 + z)4 evolution; i.e. the AGN fraction demonstrates evolution similar to the X-ray AGN luminosity function. I also find that the fraction is largest for late type, blue cloud host galaxies---however, as the majority of our galaxies are faint point sources, it is possible that AGN contamination of the host galaxy spectral energy distribution biases this result. Lastly, I employ an optical galaxy luminosity function (OLF) to weight the ChaMP/SDSS AGN fraction and generalize it over a wider range of absolute magnitudes and redshifts. Assuming no evolution in the OLF or the AGN fraction, I find that about 1 in 150 galaxies (0.66+/-0.09%) with --18 > Mi > --25 and z ≤ 0.7 host an active nucleus. |
