A new sample of X-ray selected narrow emission-line galaxies

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.

Dependence of Narrow-line Region Sizes on [O iii] Luminosity in Low-redshift Active Galactic Nuclei with Double-peaked Broad Balmer Emission Lines

“…Under the current framework of the preferred (though still being improved) unified model (Antonucci 1993;Netzer 2015), type-1 AGN and type-2 AGN have exactly the same central structures of narrow emission line regions (NLRs) and broad emission line regions (BLRs; that is, the two essential structures of AGN), although the BLRs are seriously obscured by their central dust torus, leading to no apparent broad optical emission lines being detected in type-2 AGN. The well-detected polarized broad emission lines in type-2 AGN, such as in the well-discussed results in Antonucci & Miller (1985), Tran (2001Tran ( , 2003, Nagao et al (2004), Cardamone et al (2007), Shi et al (2010), Barth et al (2014), Pons & Watson (2016), and Savic et al (2018), provide robust evidence to strongly support the hypothesis of hidden BLRs in type-2 AGN. Certainly, there is one precious subclass of true type-2 AGN: a type-2 AGN without a hidden BLR.…”

SDSS J1058+5443: A Blue Quasar without Optical/NUV Broad Emission Lines

“…Interestingly, based on the pioneering work on the study of polarized broad emission lines in type 2 AGN by Antonucci & Miller (1985), Tran (2001), Tran (2003), Nagao et al (2004), Cardamone et al (2007), and followed by Shi et al (2010), Barth et al (2014), Zhang (2014), Li et al (2015), Pons & Watson (2016), and Zhang et al (2021c), there is one special kind of AGN: true type 2 AGN (or AGN with no hidden BLRs, or BLR-less AGN or unobscured type 2 AGN in the literature) that have no expected hidden central BLRs. There are about 30 Seyfert 2 galaxies with no hidden BLRs reported in Tran (2001), andTran (2003), due to the lack of polarized broad emission lines.…”

“…A true type 2 AGN candidate, SDSS J0120, is reported by Li et al (2015), through its longterm variability properties but with no-detected broad emission lines. A small sample of true type 2 AGN candidates is reported in Pons & Watson (2016) based on their properties of unobscured X-ray emission. More recently, Zhang et al (2021c) reported the composite galaxy SDSS J1039 as a true type 2 AGN candidate based on its long-term variabilities and its apparent loss of broad optical Balmer emission lines.…”

Estimating sizes of type-2 AGN narrow-line regions from multiple survey spectra – a demonstration

“…2018to Aug. 2021, well applied to re-confirm that the SDSS J0838 is a Type-2 AGN, not a true Type-2 AGN without hidden central broad emission line regions. More discussions on properties of true type-2 AGN with apparent variabilities can be found in Barth et al (2014); Li et al (2015); Pons & Watson (2016); Zhang et al (2021c).…”