Active galactic nuclei in the Sloan Digital Sky Survey. I. Sample selection

We have compiled a large sample of low-redshift active galactic nuclei (AGNs) identified via their emission-line characteristics from the spectroscopic data of the Sloan Digital Sky Survey. Since emission lines are often contaminated by stellar absorption lines, we developed an objective and efficient method of subtracting the stellar continuum from every galaxy spectrum before making emission-line measurements. The distribution of the measured H alpha FWHM values of emission-line galaxies is strongly bimodal, with two populations separated at about 1200 km s(-1). This feature provides a natural separation between narrow-line and broad-line AGNs. The narrow-line AGNs are identified using standard emission-line ratio diagnostic diagrams. There are 1317 broad-line and 3074 narrow-line AGNs identified from about 100,000 galaxy spectra selected over 1151 deg(2). This sample is used in a companion paper to determine the emission-line luminosity function of AGNs.