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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 ActiveGalactic Nuclei (AGN) in order to clarify the link between these two types ofactivity, to explore the dependence of their features on the properties oftheir host galaxies, and to examine the role of the environment in triggeringactivity. In this first paper, we present the multiwavelength database createdfor ~150,000 SDSS galaxies with 14.5 < R < 17.5. We compare different methodsof classification for AGN activity and show that pollution of nuclear spectraby host galaxy light leads to a serious misclassification of weak-lined AGNwith increasing redshift. We develop an algorithm to correct for this bias andshow that, for a fixed host luminosity, the misclassification preferentiallyaffects redder systems, suggesting that lines in these systems are weaker.After correction for dilution, the sample displays the same fraction of AGN asin the sensitive Palomar Survey of nearby galaxies (Ho et al.). We demonstratethat spectroscopically truly passive systems show signatures of X-ray or radioAGN activity; by stacking radio images we establish the median nuclear radioluminosity of this class. While we confirm that the fraction of AGN with strong[OIII] emission decreases in denser environments, we show that, including thefaint population of AGN, the fractional abundance of AGN increases withincreasing density, suggesting that emission-line AGN activity in denserenvironments is more frequent but less intense.

Active Galaxies in the Sloan Digital Sky Survey I: The Spectroscopically Unremarkable Population of the Local Universe