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

TL;DR

This study analyzes a large sample of SDSS galaxies to improve AGN classification accuracy, revealing environmental influences on AGN activity and its relation to host galaxy properties.

Contribution

It introduces a new correction algorithm for AGN misclassification due to host galaxy light contamination in SDSS spectra.

Findings

Misclassification of weak-lined AGN increases with redshift.

Corrected AGN fraction aligns with previous surveys.

AGN activity is more frequent but less intense in denser environments.

Abstract

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.