A new sample of X-ray selected narrow emission-line galaxies. II. Looking for True Seyfert 2

Abstract

A sample of X-ray and optically selected narrow emission-line galaxies (769 sources) from the 3XMM catalogue cross-correlated with SDSS (DR9) catalogue has been studied. Narrow-emission line active galactic nuclei (AGN; type-2) have been selected on the basis of their emission line ratios and/or X-ray luminosity. We have looked for X-ray unobscured type-2 AGN. As X-ray spectra were not available for our whole sample, we have checked the reliability of using the X-ray hardness ratio (HR) as a probe of the level of obscuration and we found a very good agreement with full spectral fitting results, with only 2% of the sources with apparently unobscured HR turning out to have an obscured spectrum. Despite the fact that type-2 AGN are supposed to be absorbed based on the Unified Model, about 60% of them show no sign or very low level of X-ray obscuration. After subtraction of contaminants to the sample, that is Narrow-Line Seyfert 1 and Compton-thick AGN, the fraction of unobscured Sy2 drops to 47%. For these sources, we were able to rule out dust reddening and variability for most of them as an explanation of the absence of optical broad emission-lines. The main explanations remaining are the dilution of weak/very broad emission-lines by the host galaxy and the intrinsic absence of the broad-line region (BLR) due to low accretion rates (i.e. True Sy2). However, the number of True Sy2 strongly depends on the method used to verify the intrinsic lack of broad lines. Indeed using the optical continuum luminosity to predict the BLR properties gives a much larger fraction of True Sy2 (about 90\% of the unobscured Sy2 sample) than the use of the X-ray 2 keV luminosity (about 20%). Nevertheless the number of AGN we securely detected as True Sy2 is at least three times larger that the previously confirmed number of True Sy2.

arXiv comment10 pages, 10 figures, accepted for publication in A&A

References in corpus (67)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
• The Host Galaxies of AGN
• The Origin of the Mass--Metallicity Relation: Insights from 53,000 Star-Forming Galaxies in the SDSS
• The physical properties of star forming galaxies in the low redshift universe
• Theoretical Modeling of Starburst Galaxies
• Stellar Masses and Star Formation Histories for 10^5 Galaxies from the Sloan Digital Sky Survey
• Local Supermassive Black Holes, Relics of Active Galactic Nuclei and the X-ray Background
• Interpreting the Cosmic Infrared Background: Constraints on the Evolution of the Dust Enshrouded Star Formation Rate
• Estimating Black Hole Masses in Active Galaxies Using the Halpha Emission Line
• Observational evidence for AGN feedback in early-type galaxies
• The Relation Between Optical Extinction and Hydrogen Column Density in the Galaxy
• The Chandra Deep Field South: Optical Spectroscopy I
• The distribution of absorbing column densities among Seyfert 2 galaxies
• The AGN Obscuring Torus -- End of the "Doughnut" Paradigm?
• A 3 dimensional diagnostic diagram for Seyfert 2s: probing X-ray absorption and Compton thickness
• Dust in active nuclei. I. Evidence for "anomalous" properties
• The XMM-Newton serendipitous survey. VII. The third XMM-Newton serendipitous source catalogue
• Resolving the mid-infrared cores of local Seyferts
• The incidence of obscuration in active galactic nuclei
• Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365
• Accreting SMBHs in the COSMOS field and the connection to their host galaxies
• The HELLAS2XMM survey: IV. Optical identifications and the evolution of the accretion luminosity in the Universe
• Broad Emission Line Regions in AGN: the Link with the Accretion Power
• On the Disappearance of the Broad-Line Region in Low-Luminosity Agns
• XMM-Newton observation of the Lockman Hole II. Spectral analysis
• Testing current synthesis models of the X-ray background
• Hidden Broad Line Seyfert 2 Galaxies in the CfA and 12micron Samples
• Unabsorbed Seyfert 2 galaxies
• Exploring the Low-Mass End of The M-Sigma Relation with Active Galaxies
• The Unified Model & Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
• An Unveiling Event in the Type 2 AGN NGC4388: A Challenge for a Parsec Scale Absorber
• Reliable Identification of Compton-thick Quasars at z~2: Spitzer Mid-IR spectroscopy of HDF-oMD49
• Type 1 low z AGN. I. Emission properties
• AGN Obscuration and the Unified Model
• How complex is the obscuration in AGN? New clues from the Suzaku monitoring of the X-ray absorbers in NGC7582
• Exploring the X-ray Sky with the XMM-Newton Bright Serendipitous Survey
• On The Nature of Low Luminosity Narrow Line AGN
• Narrow-Line Seyfert 1s: 15 Years Later
• The Link Between the Hidden Broad Line Region and the Accretion Rate in Seyfert 2 Galaxies
• Why X-ray--Selected AGN Appear Optically Dull
• The XMM-Newton HBS28 sample: studying the obscuration in hard X-ray selected AGNs
• Direct Measurements of the Stellar Continua and Balmer/4000 Angstrom Breaks of Red z>2 Galaxies: Redshifts and Improved Constraints on Stellar Populations
• An XMM-Newton spectral survey of 12 micron selected galaxies. II. Implications for AGN selection and unification
• Unobscured Type 2 AGNs
• Elusive AGN in the XMM-Newton bright serendipitous survey
• A high Eddington-ratio, true Seyfert 2 galaxy candidate: implications for broad-line-region models
• Indecent Exposure in Seyfert 2 Galaxies: A Close Look
• The Hellas2XMM survey: III. Multiwavelength observations of hard X-ray selected sources in the PKS 0312-77 field
• Supermassive Black Holes and Their Host Galaxies - II. The correlation with near-infrared luminosity revisited
• Cross-correlation of the 2XMMi catalogue with Data Release 7 of the Sloan Digital Sky Survey
• On the nature of unabsorbed Seyfert 2 galaxies
• A wide search for obscured Active Galactic Nuclei using XMM-Newton and WISE
• The XMM-Newton bright serendipitous survey. Identification and optical spectral properties
• Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies
• Searching for Compton-thick active galactic nuclei at z~0.1
• Disk Outflows and High-Luminosity True Type 2 AGN
• Naked active galactic nuclei
• The Low-mass, Highly Accreting Black Hole Associated with the Active Galactic Nucleus 2XMM J123103.2+110648
• The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
• X-ray spectra of sources in the 13H XMM-Newton / Chandra deep field
• On the Lx-L6micron ratio as a diagnostic for Compton-thick AGN
• Identifying high redshift AGNs using X-ray hardness
• A new sample of X-ray selected narrow emission-line galaxies. I. The nature of optically elusive AGN
• Optical and infrared properties of active galactic nuclei in the Lockman Hole
• XMMFITCAT: The XMM-Newton spectral-fit database
• The X-ray luminous galaxies optically classified as star forming are mostly narrow line Seyfert 1s
• A unique X-ray unabsorbed Seyfert 2 galaxy IRAS F01475-0740

Cited by in corpus (9)

• Crossing the Line: Active Galactic Nuclei in the Star-forming region of the BPT Diagram
• A new sample of X-ray selected narrow emission-line galaxies. I. The nature of optically elusive AGN
• Dependence of NLRs sizes on [OIII] luminosity in low redshift AGN with double-peaked broad Balmer emission lines
• Circumnuclear regions of different BPT types in star-forming MaNGA galaxies: AGN detectability
• SDSS J1058+5443: A blue quasar does not have optical/NUV broad emission lines
• Systematic research of low redshift optically selected SDSS Type-2 AGN but with apparent long-term optical variabilities from Catalina Sky Survey, I: data sample and basic results
• Long-term variability of the composite galaxy SDSS J103911-000057: A candidate of true Type-2 AGN
• VLT-MUSE spectroscopy of AGNs misclassified by BPT diagnostic or with weak emission lines
• Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations