THE ENVIRONMENT OF ACTIVE GALACTIC NUCLEI IN THE SLOAN DIGITAL SKY SURVEY

摘要

We present the observed fraction of galaxies with an active galactic nucleus (AGN) as a function of environment in the early data release of the Sloan Digital Sky Survey (SDSS). Using 4921 galaxies in the redshift range 0.05 ≤ z ≤ 0.095 and brighter than M(r~*) = -20.0 (or M~* + 1.45), we find at least ～20% of these galaxies possess an unambiguous detection of an AGN, but this fraction could be as high as approx= 40% after we model the ambiguous emission-line galaxies in our sample. We have studied the environmental dependence of galaxies, using the local galaxy density as determined from the distance to the 10th nearest neighbor. As expected, we observe that the fraction of star-forming galaxies decreases with density, while the fraction of passive galaxies (no emission lines) increases with density. In contrast, the fraction of galaxies with an AGN remains constant from the cores of galaxy clusters to the rarefied field population. We conclude that the presence of an AGN is independent of the disk component of a galaxy. We have extensively tested our results, and they are robust against measurement error, definition of an AGN, aperture bias, stellar absorption, survey geometry, and signal-to-noise ratio. Our observations are consistent with the hypothesis that a supermassive black hole resides in the bulge of all massive galaxies, and approx= 40% of these black holes are seen as AGNs in our sample. A high fraction of local galaxies with an AGN suggests either that the mean lifetime of these AGNs is longer than previously thought (i.e., ≥10~8 yr) or that the AGNs burst more often than expected: ～40 times over the redshift range of our sample.