The HELLAS2XMM survey - VIII. Optical identifications of the extended sample

Astronomy & Astrophysics, Apr 2007

Aims.Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN.Methods.We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. Their 2–10 keV fluxes cover the range 6    10-13  and the total area surveyed is ~0.5 deg2 at the bright flux limit. We spectroscopically identified 59 new sources, bringing the spectroscopic completeness of the full HELLAS2XMM sample to almost 70% over a total area of ~1.4 deg2 at the bright flux limit. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to . We measured the flux and luminosity of the [OIII] emission line for 59 of these sources.Results.Assuming that most high X-ray-to-optical flux ratio sources are obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their . We find obscured QSOs surface density of 45 ± 15 and 100–350 deg-2 down to flux limits of 10-14 and 10-15 , respectively. At these flux limits, the fraction of X-ray-selected obscured QSOs turns out to be similar to that of unobscured QSOs. Since X-ray selection misses most Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs. 
We find that hard X-ray selected AGNs with a detected [OIII] emission span a wide range of with a logarithmic median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of optically selected AGNs (median 1.69 and interquatile range 0.30), suggesting that optically selected samples are at least partly incomplete andor that [OIII] emission is not a perfect isotropic indicator of the nuclear power. The seven X-ray bright, optically normal galaxy (XBONG) candidates in the sample have , while their X-ray and optical luminosities and obscuring column density are similar to those of narrow-line AGNs in the same redshift interval (0.075–0.32). This suggests that, while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured.

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The HELLAS2XMM survey - VIII. Optical identifications of the extended sample

A&A Astronomy & Astrophysics F. Cocchia 0 1 3 F. Fiore 3 C. Vignali 6 7 M. Mignoli 6 M. Brusa 5 A. Comastri 6 C. Feruglio 1 3 A. Baldi 4 N. Carangelo 9 P. Ciliegi 6 V. D'Elia 3 F. La Franca 8 R. Maiolino 2 3 G. Matt 8 S. Molendi 9 G. C. Perola 8 S. Puccetti 3 10 0 INAF - Osservatorio Astronomico di Brera , via Brera 28, 20121 Milano , Italy 1 Dipartimento di Fisica, Università di Roma Tor Vergata , via della Ricerca Scientifica 1, 00133 Rome , Italy 2 INAF - Osservatorio Astrofisico di Arcetri , Largo Enrico Fermi 5, 50125 Firenze , Italy 3 INAF - Osservatorio Astronomico di Roma , via Frascati 33, Monteporzio-Catone (RM) 00040 , Italy 4 Harvard-Smithsonian Center for Astrophysics (CfA) , 60 Garden str, Cambridge 02138 MA , USA 5 Max Planck Institüt für Extraterrestrische Physik (MPE) , Giessenbachstr. 1, 85748 Garching , Germany 6 INAF - Osservatorio Astronomico di Bologna , via Ranzani 1, 40127 Bologna , Italy 7 Dipartimento di Astronomia, Università di Bologna , via Ranzani 1, 40127 Bologna , Italy 8 Dipartimento di Fisica, Università Roma Tre , via della Vasca Navale 84, 00146 Roma , Italy 9 INAF - IASF , via Bassini 15, 20133 Milano , Italy 10 ASI Science Data Center, ASDC c/o ESRIN , via G. Galilei, 00044 Frascati , Italy Aims. Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN. Methods. We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. Their 2-10 keV fluxes cover the range 6 × 10−15−4 × 10−13 erg cm−2 s−1 and the total area surveyed is ∼0.5 deg2 at the bright flux limit. We spectroscopically identified 59 new sources, bringing the spectroscopic completeness of the full HELLAS2XMM sample to almost 70% over a total area of ∼1.4 deg2 at the bright flux limit. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to R ∼ 25. We measured the flux and luminosity of the [OIII]λ5007 emission line for 59 of these sources. Results. Assuming that most high X-ray-to-optical flux ratio sources are obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their log N− log S . We find obscured QSOs surface density of 45 ± 15 and 100-350 deg−2 down to flux limits of 10−14 and 10−15 erg cm−2 s−1, respectively. At these flux limits, the fraction of X-ray-selected obscured QSOs turns out to be similar to that of unobscured QSOs. Since X-ray selection misses most Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs. We find that hard X-ray selected AGNs with a detected [OIII] emission span a wide range of L2−10 keV/L[OIII] with a logarithmic median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of optically selected AGNs (median 1.69 and interquatile range 0.30), suggesting that optically selected samples are at least partly incomplete and/or that [OIII] emission is not a perfect isotropic indicator of the nuclear power. The seven X-ray bright, optically normal galaxy (XBONG) candidates in the sample have L2−10 keV/L[OIII] ∼> 1000, while their X-ray and optical luminosities and obscuring column density are similar to those of narrow-line AGNs in the same redshift interval (0.075-0.32). This suggests that, while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured. X-rays; diffuse background - surveys - galaxies; active - galaxies; evolution 1. Introduction Deep Chandra and XMM-Newton hard X-ray surveys have been able to detect the sources making up the majority of the cosmic Based on observations collected at the European Southern Observatory, Prog. ID 67.A-0401, 68.A-0514, 69.A-0563 and 072.A0633, and at the Telescopio Nazionale Galileo, Prog. ID 1_15_083. Based also on observations made with XMM-Newton, an ESA science mission. Tables 1, 3, 4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/31 X-ray background (XRB) below 6–7 keV (Giacconi et al. 2002; Bauer et al. 2004; Moretti et al. 2003; Worsley et al. 2004; Brandt & Hasinger 2005) . However, deep surveys cover only a fraction of a square degree of sky, making it difficult to find sizable samples of medium- and high-luminosity sources. To obtain a more complete coverage of the redshift-luminosity plane, which complements deep surveys, and to compute an accurate luminosity function over wide luminosity and redshift intervals a much larger area on the order of a few square degrees needs to be covered. Furthermore, large-area surveys can provide sizeable samples of “rare” objects. To these purposes, we One of the most interesting (...truncated)


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F. Cocchia, F. Fiore, C. Vignali, M. Mignoli, M. Brusa, A. Comastri, C. Feruglio, A. Baldi, N. Carangelo, P. Ciliegi, V. D'Elia, F. La Franca, R. Maiolino, G. Matt, S. Molendi, G. C. Perola, S. Puccetti. The HELLAS2XMM survey - VIII. Optical identifications of the extended sample, Astronomy & Astrophysics, 2007, pp. 31-40, Volume 466, Issue 1, DOI: 10.1051/0004-6361:20065170