XMM-NEWTONOBSERVATIONS OF A COMPLETE SAMPLE OF OPTICALLY SELECTED TYPE 2 SEYFERT GALAXIES

Abstract

The majority of Active Galactic Nuclei (AGN) suffer from significant obscuration by surrounding dust and gas. The penetrating power and sensitivity of hard X-ray observations allows obscured AGN to be probed out to high redshifts.However, X-ray surveys in the 2-10 keV band will miss the most heavily-obscured AGN in which the absorbing column density exceeds ∼ 10 24 cm −2 (the Comptonthick AGN). It is therefore vital to know the fraction of AGN that are missed in such X-rays surveys and to determine if these AGN represent some distinct population in terms of the fundamental properties of AGN and/or their host galaxies. In this paper we present the analysis of XMM-Newton X-ray data for a complete sample of 17 low-redshift Type 2 Seyfert galaxies chosen from the Sloan Digital Sky Survey based solely on the high observed flux of the [OIII]λ5007 emission-line. This line is formed in the Narrow Line Region hundreds of parsecs away from the central engine. Thus, unlike the X-ray emission, it is not affected by obscuration due to the torus surrounding the black hole. It therefore provides a useful isotropic indicator of the AGN luminosity. As additional indicators of the intrinsic AGN luminosity, we use the Spitzer Space Telescope to measure the luminosities of the mid-infrared continuum and the [OIV]25.89µm narrow emission-line. We then use the ratio of the 2-10 keV X-ray luminosity to the [OIII], [OIV], and mid-infrared luminosities to assess the amount of Xray obscuration and to distinguish between Compton-thick and Compton-thin objects. The various diagnostics of AGN luminosity with heavily obscured hard X-ray emission are in broad agreement. We find that the majority of the sources suffer significant amounts of obscuration: the observed 2-10 keV emission is depressed by more than an order-of-magnitude in 11 of the 17 cases (as expected for Compton-thick sources). Thus, surveys in the rest-frame 2-10 keV band will be significantly incomplete for obscured AGN. We find a strong inverse correlation between the ratio of the 2-10 keV X-ray and [OIII] (or [OIV] or mid-IR) fluxes

The X-ray emission of local luminous infrared galaxies

“…6.2) and therefore decreases the observed EW of the Fe Kα emission line. To estimate the AGN X-ray luminosity of these objects, we used the flux of the Fe Kα emission line because it seems to be a good indicator of the intrinsic AGN luminosity (Ptak et al 2003;Levenson et al 2006;LaMassa et al 2009). We assumed L FeKα /L int 2−10 keV = 2 × 10 −3 (Levenson et al 2006).…”

The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$

“…For example, Compton thick AGN may be missed by X-ray surveys (La Massa et al 2009), while the optical selection can fail to select AGN light diluted by their host galaxy. Moreover, the redshift range covered by X-ray samples usually extends to higher redshift than the optical ones.…”

The Optx Project. Iv. How Reliable Is [O Iii] as a Measure of Agn Activity?

“…Since the narrow emission-line region lies outside of the dusty torus, the emission lines should not suffer from obscuration by that high column density material (though they may be affected by dust within the host galaxy). Although there is some evidence that the [O iii] luminosity is a good measure of AGN activity (e.g., Mulchaey et al 1994), and, indeed, many researchers have assumed it to be so in their analyses (e.g., Alonso-Herrero et al 1997;Hao et al 2005;Netzer et al 2006;Bongiorno et al 2010), recent work has called this conclusion into question (e.g., Cocchia et al 2007;Meléndez et al 2008;Diamond-Stanic et al 2009;La Massa et al 2009;Winter et al 2010).…”

“…With the advent of the Chandra and XMM-Newton X-ray observatories, AGN activity can now also be traced through the 2-10 keV luminosity, although this measurement may suffer from some absorption (e.g., Diamond-Stanic et al 2009;La Massa et al 2009). Also, X-ray selection misses the most heavily absorbed, Compton-thick sources with N H > 10 24 cm −2 (see Comastri 2004 for a review).…”

“…In this paper, we explore how well the BPT diagram does in classifying as AGNs the sources in the OPTX X-rayselected AGN sample and how that classification relates to the very broad observed range in L 2-10 keV /L [O iii] (e.g., Cocchia et al 2007;Meléndez et al 2008;La Massa et al 2009). Following Heckman et al (2005), Bongiorno et al (2010), and Georgantopoulos & Akylas (2010), we use our measurements of the mean luminosity ratios to transform the fits to the OPTX+Swift BAT hard X-ray luminosity function (LF) from Yencho et al (2009) and the local RXTE hard X-ray LF from Sazonov & Revnivtsev (2004) Hao et al (2005) and Reyes et al (2008) to investigate the efficiency of optical versus X-ray selection.…”