Active galaxies

While extragalactic objects such as normal and active galaxies are certainly the most EUV luminous sources discovered, they are also among the most difficult to observe. Their great distance, coupled with the absorbing effect of the intergalactic medium and the interstellar gas in our own galaxy, yields fluxes fainter than most of the more local EUV sources. Hence, the acquisition of EUV spectra requires comparatively long exposure times. With the capability of EUVE, typical minimum exposure times were a few hundred thousand seconds, approaching the practical limit imposed by the instrument background, beyond which no further improvement in signal-to-noise could be achieved. Consequently, the number of extragalactic objects for which spectroscopic observations have been feasible is small. Furthermore, these sources are only visible in the short wavelength region of the EUVE short wavelength spectrometer. Table 10.1 lists those objects which have published EUV spectra, noting their exposure times and classification. Two BL Lac objects and 5 Seyfert galaxies, probably all type I, are listed.

The now commonly accepted explanation of the various different types of AGN is the so-called ‘Unified Model’, which adopts a common physical mechanism for the source, the AGN types representing different viewing angles. In this model, the central energy source is a massive black hole accreting matter from its host galaxy via a disc. This disc is surrounded by a thick torus of material as shown in figure 10.1.