Active Galactic Nuclei in the Sloan Digital Sky Survey. I. Sample Selection

Abstract

We have compiled a large sample of low-redshift active galactic nuclei (AGN)\nidentified via their emission line characteristics from the spectroscopic data\nof the Sloan Digital Sky Survey. Since emission lines are often contaminated by\nstellar absorption lines, we developed an objective and efficient method of\nsubtracting the stellar continuum from every galaxy spectrum before making\nemission line measurements. The distribution of the measured H$\\alpha$ Full\nWidth at Half Maxima values of emission line galaxies is strongly bimodal, with\ntwo populations separated at about 1,200km s$^{-1}$. This feature provides a\nnatural separation between narrow-line and broad-line AGN. The narrow-line AGN\nare identified using standard emission line ratio diagnostic diagrams. 1,317\nbroad-line and 3,074 narrow-line AGN are identified from about 100,000 galaxy\nspectra selected over 1151 square degrees. This sample is used in a companion\npaper to determine the emission-line luminosity function of AGN.\n