The deep Groth Strip Survey. VI. Spectroscopic, variability, and X-ray detection of active galactic nuclei

We identify active galactic nuclei (AGNs) in the Groth-Westphal Survey Strip (GSS) using the independent and complementary selection techniques of optical spectroscopy and photometric variability. We discuss the X-ray properties of these AGNs using Chandra/XMM-Newton data for this region. From a sample of 576 galaxies with high-quality spectra we identify 31 galaxies with AGN signatures. Seven of these have broad emission lines (Type 1 AGNs). We also identify 26 galaxies displaying nuclear variability in Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images of the GSS separated by similar to 7 yr. The primary overlap of the two selected AGN samples is the set of broad-line AGNs, of which 80% appear as variable. Only a few narrow-line AGNs approach the variability threshold. The broad-line AGNs have an average redshift of < z > similar or equal to 1.1, while the other spectroscopic AGNs have redshifts closer to the mean of the general galaxy population (< z > similar or equal to 0: 7). Eighty percent of the identified broad-line AGNs are detected in X-rays, and these are among the most luminous X-ray sources in the GSS. Only one narrow-line AGN is X-ray detected. Of the variable nuclei galaxies within the X-ray survey, 27% are X-ray detected. We find that 1.9% +/- 0.6% of GSS galaxies to V-gal = 24 are broad-line AGNs, 1.4% +/- 0.5% are narrow-line AGNs, and 4.5% +/- 1.4% contain variable nuclei. The fraction of spectroscopically identified BLAGNs and NLAGNs at z similar to 1 reveals a marginally significant increase of 1.3% +/- 0.9% when compared to the local population.