Obscured AGN at 1.5 < z < 3.0 from the zCOSMOS-deep Survey

I. Properties of the emitting gas in the narrow-line region^⋆

¹ INAF – Osservatorio di Astrofisica e Scienza delle Spazio di Bologna, OAS, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: marco.mignoli@inaf.it
² Sorbonne Universités, UPMC-CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
³ Université Lyon, Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR 5574, 69230 Saint-Genis-Laval, France
⁴ SISSA, Via Bonomea 265, 34136 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Roma, 00040 Monteporzio Catone, Italy
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, 40127 Bologna, Italy
⁷ Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁸ Aix Marseille Université, CNRS, LAM, UMR 7326, 13388 Marseille, France
⁹ MIT-Kavli Center for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹⁰ Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹³ Institute of Astronomy, Madingley Road Cambridge, CB3 0HA Cambridge, UK
¹⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via Bassini 15, 20133 Milan, Italy
¹⁵ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁶ University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁸ Kavli Institute for Astronomy & Astrophysics, Peking University, 5 Yiheyuan Road, Beijing 100871, PR China
¹⁹ Instituto de Astrofisica de Andalucia, CSIC, Apartado de correos 3004, 18080 Granada, Spain
²⁰ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

Received: 15 January 2019
Accepted: 27 March 2019

Abstract

Context. The physics and demographics of high-redshift obscured active galactic nuclei (AGN) is still scarcely investigated. New samples of such objects, selected with different techniques, can provide useful insights into their physical properties.

Aims. With the goal to determine the properties of the gas in the emitting region of type 2 AGN, in particular, the gas metal content, we exploit predictions from photoionization models, including new parameterizations for the distance of gas distribution from the central source and internal microturbulence in the emitting clouds, to interpret rest-frame UV spectral data.

Methods. We selected a sample of 90 obscured (type 2) AGN with 1.45 ≤ z ≤ 3.05 from the zCOSMOS-deep galaxy sample by 5σ detection of the high-ionization C IV λ1549 narrow emission line. This feature in a galaxy spectrum is often associated with nuclear activity, and the selection effectiveness has also been confirmed by diagnostic diagrams based on utraviolet (UV) emission-line ratios. We applied the same selection technique and collected a sample of 102 unobscured (type 1) AGN. Taking advantage of the large amount of multiband data available in the COSMOS field, we investigated the properties of the C IV-selected type 2 AGN, focusing on their host galaxies, X-ray emission, and UV emission lines. Finally, we investigated the physical properties of the ionized gas in the narrow-line region (NLR) of this type 2 AGN sample by combining the analysis of strong UV emission lines with predictions from photoionization models.

Results. We find that in order to successfully reproduce the relative intensity of UV emission lines of the selected high-z type 2 AGN, two new ingredients in the photoionization models are fundamental: small inner radii of the NLR (≈90 pc for L_AGN = 10⁴⁵ erg s⁻¹), and the internal dissipative microturbulence of the gas-emitting clouds (with v_micr ≈ 100 km s⁻¹). With these modified models, we compute the gas-phase metallicity of the NLR, and our measurements indicate a statistically significant evolution of the metal content with redshift. Finally, we do not observe a strong relationship between the NLR gas metallicity and the stellar mass of the host galaxy in our C IV-selected type 2 AGN sample.