  (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).

Cosmology and Nongalactic Astrophysics