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 between 0.05 \leq z \leq 0.095 , and brighter than { M _ { r ^ { * } } } = -20.0 ( or { M ^ { \star } } +1.45 ) , we find at least \sim 20 \% of these galaxies possess an unambiguous detection of an AGN , but this fraction could be as high as \simeq 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 10 ^ { th } 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 .
Our observations are consistent with the hypothesis that a supermassive black hole resides in the bulge of all massive galaxies and \simeq 40 \% of these black holes are seen as AGNs in our sample .
A high fraction of local galaxies with an AGN suggests that either the mean lifetime of these AGNs is longer than previously thought ( i.e . , \geq 10 ^ { 8 } years ) , or that the AGN burst more often than expected ; \sim 40 times over the redshift range of our sample .