Obscured AGN at z ~ 1 from the zCOSMOS-Bright Survey

I. Selection and optical properties of a [Ne v]-selected sample

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: marco.mignoli@oabo.inaf.it
² Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
³ INAF – Osservatorio Astronomico di Roma, 00040 Monteporzio Catone, Italy
⁴ Institut de Recherche en Astrophysique et Planétologie, CNRS, 31400 Toulouse, France
⁵ IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
⁶ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁷ Institute of Astronomy, ETH Zurich, 8093 Zürich, Switzerland
⁸ Laboratoire d’Astrophysique de Marseille, Aix-Marseille Université, CNRS, Marseille, France
⁹ European Southern Observatory, Garching, Germany
¹⁰ INAF – Osservatorio Astronomico di Padova, Padova, Italy
¹¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy
¹² Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
¹³ INAF – Osservatorio Astronomico di Brera, Milano, Italy
¹⁴ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, EH93 HJ, UK
¹⁵ University of Vienna, Department of Astronomy, 1180 Vienna, Austria
¹⁶ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, 277-8583 Kashiwa, Japan
¹⁷ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Italy
¹⁸ Institut d’Astrophysique de Paris, Université Pierre & Marie Curie, 75014 Paris, France
¹⁹ Institut d’Astrophysique Spatiale, Bâtiment 121, CNRS & Univ. Paris Sud XI, 91405 Orsay Cedex, France

Received: 4 December 2012
Accepted: 25 May 2013

Abstract

Aims. The application of multi-wavelength selection techniques is essential for obtaining a complete and unbiased census of active galactic nuclei (AGN). We present here a method for selecting z ~   1 obscured AGN from optical spectroscopic surveys.

Methods. A sample of 94 narrow-line AGN with 0.65 < z < 1.20 was selected from the 20k-Bright zCOSMOS galaxy sample by detection of the high-ionization [Ne v] λ3426 line. The presence of this emission line in a galaxy spectrum is indicative of nuclear activity, although the selection is biased toward low absorbing column densities on narrow-line region or galactic scales. A similar sample of unobscured (type 1 AGN) was collected applying the same analysis to zCOSMOS broad-line objects. This paper presents and compares the optical spectral properties of the two AGN samples. Taking advantage of the large amount of data available in the COSMOS field, the properties of the [Ne v]-selected type 2 AGN were investigated, focusing on their host galaxies, X-ray emission, and optical line-flux ratios. Finally, a previously developed diagnostic, based on the X-ray-to-[Ne v] luminosity ratio, was exploited to search for the more heavily obscured AGN.

Results. We found that [Ne v]-selected narrow-line AGN have Seyfert 2-like optical spectra, although their emission line ratios are diluted by a star-forming component. The ACS morphologies and stellar component in the optical spectra indicate a preference for our type 2 AGN to be hosted in early-type spirals with stellar masses greater than 10^9.5 − 10   M_⊙, on average higher than those of the galaxy parent sample. The fraction of galaxies hosting [Ne v]-selected obscured AGN increases with the stellar mass, reaching a maximum of about 3% at ≈2 × 10¹¹   M_⊙. A comparison with other selection techniques at z ~ 1, namely the line-ratio diagnostics and X-ray detections, shows that the detection of the [Ne v] λ3426 line is an effective method for selecting AGN in the optical band, in particular the most heavily obscured ones, but cannot provide a complete census of type 2 AGN by itself. Finally, the high fraction of [Ne v]-selected type 2 AGN not detected in medium-deep (≈100–200 ks) Chandra observations (67%) is suggestive of the inclusion of Compton-thick (i.e., with N_H > 10²⁴  cm^-2) sources in our sample. The presence of a population of heavily obscured AGN is corroborated by the X-ray-to-[Ne v] ratio; we estimated, by means of an X-ray stacking technique and simulations, that the Compton-thick fraction in our sample of type 2 AGN is 43 ± 4% (statistical errors only), which agrees well with standard assumptions by XRB synthesis models.