The optical-ultraviolet continuum of Seyfert 2 galaxies

This paper aims to understand the continuum of Seyfert 2 galaxies. By fitting the single galaxies in the sample of Heckman et al. with composite models (shock + photoionization), we show that five main components characterize the spectral energy distribution (SED) of the continuum. Emission in the radio range can be recognized as bremsstrahlung from relatively cold gas or synchrotron radiation due to Fermi mechanism at the shock front. The bump in the IR is due to reradiation of the central radiation by dust and to mutual heating and cooling between dust and gas. In the optical-UV range, the main component is due to bremsstrahlung from gas heated and ionized by the primary flux (the flux from the active center [AC] and/or the radiation from young stars), by diffuse radiation emitted by the hot slabs of gas, and by collisional ionization. Shocks play an important role since they produce a high-temperature zone where soft X-rays are emitted. Finally, the harder X-ray radiation from the AC, seen through the clouds, is easily recognizable in the spectrum. Assuming that the narrow-line region (NLR) is powered by a power-law central ionizing radiation and by shocks, we discuss the optical-UV featureless continuum of Seyfert 2. We show that in this wavelength range the slope of the NLR emission reproduces the observed values and may be the main component of the featureless continuum. However, the presence of star-forming regions cannot be excluded in the circumnuclear region of various Seyfert galaxies. Their photoionizing radiation may prevail in the outskirts of the galaxy where the power-law radiation from the AC is diluted. An attempt is made to find their fingerprints in the observed active galactic nucleus spectra. Finally, it is demonstrated that multicloud models are necessary to interpret the spectra of single objects, even in the global investigation of a sample of galaxies.