The space density of Compton-thick AGN at z ≈ 0.8 in the zCOSMOS-Bright Survey
Dipartimento di Fisica e AstronomiaUniversità degli Studi di
Viale Berti Pichat 6/2,
2 INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
3 ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
4 European Southern Observatory, Karl-Schwarzschild-Straβe 2, 85748 Garching, Germany
5 INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
Received: 11 August 2014
Accepted: 17 September 2014
Context. The obscured accretion phase in black hole growth is a crucial ingredient in many models linking the active galactic nuclei (AGN) activity with the evolution of their host galaxy. At present, a complete census of obscured AGN is still missing, although several attempts in this direction have been carried out recently, mostly in the hard X-rays and at mid-infrared wavelengths.
Aims. The purpose of this work is to assess whether the [Ne v] emission line at 3426 Å can reliably pick up obscured AGN up to z ≈ 1 by assuming that it is a reliable proxy of the intrinsic AGN luminosity and using moderately deep X-ray data to characterize the amount of obscuration.
Methods. A sample of 69 narrow-line (Type 2) AGN at z ≈ 0.65–1.20 were selected from the 20k-zCOSMOS Bright galaxy sample on the basis of the presence of the [Ne v]3426 Å emission. The X-ray properties of these galaxies were then derived using the Chandra-COSMOS coverage of the field; the X-ray-to-[Ne v] flux ratio, coupled with X-ray spectral and stacking analyses, was then used to infer whether Compton-thin or Compton-thick absorption is present in these sources. Then the [Ne v] luminosity function was computed to estimate the space density of Compton-thick AGN at z ≈ 0.8.
Results. Twenty-three sources were detected by Chandra, and their properties are consistent with moderate obscuration (on average, ≈a few × 1022 cm-2). The X-ray properties of the remaining 46 X-ray undetected Type 2 AGN (among which we expect to find the most heavily obscured objects) were derived using X-ray stacking analysis. Current data, supported by Monte Carlo simulations, indicate that a fraction as high as ≈40% of the present sample is likely to be Compton thick. The space density of Compton-thick AGN with logL2−10 keV> 43.5 at z = 0.83 is ΦThick = (9.1 ± 2.1) × 10-6 Mpc-3, in good agreement with both X-ray background model expectations and the previously measured space density for objects in a similar redshift and luminosity range. We regard our selection technique for Compton-thick AGN as clean but not complete, since even a mild extinction in the narrow-line region can suppress [Ne v] emission. Therefore, our estimate of their space density should be considered as a lower limit.
Key words: galaxies: nuclei / quasars: emission lines / quasars: general / X-rays: galaxies / X-rays: general
© ESO, 2014