| Summary: | Near near-infrared spectroscopic observations of several Active Galactic Nuclei (<i>AGN</i>) are presented. In combination with existing data on X-ray, visible and far-infrared frequencies, these data are used to investigate the effects of interstellar extinction on their observed spectral energy distributions. The derived intrinsic properties of these sources are compared in detail with those expected in terms of current 'unified schemes' of AGN, which seek to relate the various observational classes through the parameters of luminosity, viewing angle and obscuration. After first presenting the methods of data reduction and analysis appropriate to infrared spectra, I use observations of the Paschen β hydrogen recombination line in three so-called <i>Narrow Line X-ray Galaxies</i> to show that their <i>intrinsic</i> (extinction-corrected) broad-line and X-ray properties can account for in terms of normal Seyfert 1 nuclei obscured by moderate interstellar extinction along our line-of-sight. Near-infrared spectroscopy of the luminous radio galaxy Cygnus A at λ≈2 μm reveals strong narrow line emission from excited H_2, [SivI] and H^+ , which I argue is excited by a highly-obscured quasar nucleus. Using the correlations established for normal quasars I derive an extinction of <i>A</i>_v = 37±7 mag towards this nucleus, and show that wavelength-independent electron scattering of nuclear radiation plausibly accounts for the observed optical and near-infrared non-stellar continuum. I also demonstrate that the quasar nucleus is also able to power the observed far-infrared continuum through heating of interstellar dust by the (unobserved) ultraviolet continuum. Similar observations of the infrared-luminous galaxy merger Markarian 463 are presented as part of a multiwavelength study to determine the origin of the high far-infrared luminosities of such systems. In Markarian 463 this luminosity is provided almost wholly by a Seyfert nucleus which is obscured from direct view at optical wavelengths, but which I predict should be detectable as a hard X-ray source. Using ultraviolet, optical and infrared line observations I am also able to estimate the quantity and location of the obscuring dust towards the line-emitting regions in Markarian 463(E). I discuss the roles of various emission mechanisms and the effects of extinction and scattering on the appearance of Markarian 463(E) from hard X-ray to sub-millimetre frequencies.
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