XMM-Newton Observations of a Complete Sample of Optically Selected Type 2 Seyfert Galaxies

TL;DR

This study uses XMM-Newton X-ray data and isotropic indicators like [OIII], [OIV], and mid-infrared luminosities to identify heavily obscured, Compton-thick AGN in a complete sample of optically selected Type 2 Seyfert galaxies.

Contribution

It provides a systematic analysis of obscuration in Type 2 Seyfert galaxies using multiple luminosity indicators to identify Compton-thick AGN.

Findings

Majority of sources are heavily obscured with depressed X-ray emission.

11 out of 17 sources are likely Compton-thick AGN.

X-ray surveys may miss a significant fraction of obscured AGN.

Abstract

(abridged)The majority of Active Galactic Nuclei (AGN) suffer from significant obscuration by surrounding dust and gas. X-ray surveys in the 2-10 keV band will miss the most heavily-obscured AGN in which the absorbing column density exceeds $\sim10^{24}$cm$^{-2}$ (the Compton-thick 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 \textit{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]$\lambda$5007 emission-line. This line is formed in the Narrow Line Region hundreds of parsecs away from the central engine. Thus, unlike the X-ray emission, it is not affected by obscuration due to the torus surrounding the black hole. It therefore provides a useful isotropic indicator of the AGN luminosity. As additional indicators of the intrinsic AGN luminosity, we use the Spitzer Space Telescope to measure the luminosities of the mid-infrared continuum and the [OIV]25.89$\mu$m 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 X-ray obscuration and to distinguish between Compton-thick and Compton-thin objects. 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).