HARD X-RAY-EMISSION FROM EXTRAGALACTIC IRAS 12-MICRON SOURCES - CONSTRAINTS ON THE UNIFIED ACTIVE GALACTIC NUCLEUS MODEL AND THE SYNTHESIS OF THE X-RAY-BACKGROUND

We analyze the 2-10 keV X-ray emission of recently compiled complete samples of active galactic nuclei (AGNs) and galaxies selected at 12 mu m. The content in active galaxies of the 12 mu m sample is indeed much larger and less biased against low-luminosity and ''hidden'' active nuclei than samples selected at other wavelengths. As a necessary complement, we also study the IR emissivity of the largest sample of hard-X-ray-selected AGNs. Our purposes are to probe the unified scheme of active nuclei and to evaluate the local X-ray volume emissivity of low-luminosity and ''hidden'' AGNs. Two methods are used in our analysis to search for X-ray emission in the HEAO I A-2 all-sky maps from the extragalactic sources contained in the extended 12 mu m sample: an X-ray intensity distribution analysis around the known positions of the sources and a cross-correlation function analysis between galaxy densities and X-ray intensities in the high Galactic latitude sky (\b\ > 25 degrees), where the catalog is more than 98% complete. The joint analysis of the X-ray-selected and 12 mu m-selected samples enables us to define the relationship between the X-ray and the mid-IR emissions of Seyfert nuclei. In particular, we find congruence between the hard X-ray and mid-IR Seyfert 1 nuclear-luminosity functions when a nonlinear relation L(12 mu m) proportional to L(5 keV)(0.8) is with appropriate dispersion, Although this result is easily explained by circumnuclear reradiation as the main source of the mid-IR emission, it nonetheless argues against the simplest version of the unified model for active nuclei. We confirm that X-ray emission is basically restricted to AGNs, finding Seyfert 1's to be much more powerful emitters than Seyfert 2's. We find no signal of X-ray emission for the remaining galaxies selected at 12 mu m, not even for those having a flat far-IR spectrum, which are prime candidates to host hidden AGNs. Our result on the local volume emissivity of the Seyfert 1's is fully consistent with those that we derive from studies of the hard X-ray luminosity function, The 95% upper limits to the local emissivity that we derive for the Seyfert 2 class and for the most likely ''hidden'' AGN candidates are consistent with models of the X-ray background recently proposed based on the unified Seyfert scheme. We also find that less than similar to 20% of Seyfert 1-like AGNs and less than similar to 50% of Seyfert 2-like AGNs can be ''hidden'' in the non-Seyfert-galaxy samples, assuming that the X-ray-to-12 mu m flux ratios measured in the 12 mu m-selected samples can be considered typical for these classes of objects. In this case, the number ratio of Seyfert 2 to Seyfert 1 galaxies ranges from 1 to 2.