Active Galaxies in the Sloan Digital Sky Survey I: The Spectroscopically
  Unremarkable Population of the Local Universe
  In a series of papers based on the FIRST and Sloan Digital Sky Surveys
(SDSS), we investigate the local population of star-forming galaxies and Active
Galactic Nuclei (AGN) in order to clarify the link between these two types of
activity, to explore the dependence of their features on the properties of
their host galaxies, and to examine the role of the environment in triggering
activity. In this first paper, we present the multiwavelength database created
for ~150,000 SDSS galaxies with 14.5 < R < 17.5. We compare different methods
of classification for AGN activity and show that pollution of nuclear spectra
by host galaxy light leads to a serious misclassification of weak-lined AGN
with increasing redshift. We develop an algorithm to correct for this bias and
show that, for a fixed host luminosity, the misclassification preferentially
affects redder systems, suggesting that lines in these systems are weaker.
After correction for dilution, the sample displays the same fraction of AGN as
in the sensitive Palomar Survey of nearby galaxies (Ho et al.). We demonstrate
that spectroscopically truly passive systems show signatures of X-ray or radio
AGN activity; by stacking radio images we establish the median nuclear radio
luminosity of this class. While we confirm that the fraction of AGN with strong
[OIII] emission decreases in denser environments, we show that, including the
faint population of AGN, the fractional abundance of AGN increases with
increasing density, suggesting that emission-line AGN activity in denser
environments is more frequent but less intense.
