2009ApJ...705..568L

The majority of active galactic nuclei (AGNs) suffer from significant obscuration by surrounding dust and gas. The penetrating power and sensitivity of hard X-ray observations allow obscured AGNs to be probed out to high redshifts. However, X-ray surveys in the 2-10 keV band will miss the most heavily obscured AGNs in which the absorbing column density exceeds ∼10²⁴/cm2 (the Compton-thick AGN). It is, therefore, vital to know the fraction of AGNs that are missed in such X-ray surveys and to determine if these AGNs represent some distinct population in terms of the fundamental properties of AGNs 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 [O III]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 [O IV] 25.89 µm narrow emission line. We then use the ratio of the 2-10 keV X-ray luminosity to the [O III], [O IV], and mid-infrared luminosities to assess the amount of X-ray 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 AGNs. We find a strong inverse correlation between the ratio of the 2-10 keV X-ray and [O III] (or [O IV] or mid-IR) fluxes and the equivalent width of the 6.4 keV Fe Kα line. This demonstrates that the weak hard X-ray continuum emission is due to obscuration (rather than due to intrinsically weak emission). In several cases, the large amount of obscuration is not consistent with the values of absorbing column density derived from simple spectral fits to the data. We run simulations of a more physically realistic model with partial covering of the X-ray source plus Compton scattering, and show that such models are consistent with the data. We show that the distribution of obscuration in the 2-10 keV band in our sample is similar to what is seen in samples selected in the Swift BAT energy band (14-195 keV). This implies that the BAT surveys do recover a significant fraction of the local population of Compton-thick AGNs. Finally, we find no relationship between the amount of X-ray obscuration and the other properties of the AGN and its host galaxy. Hence, Compton-thick and Compton-thin sources do not seem to trace distinct populations.