1.1 Quasars and Seyfert 1 Galaxies
These are the most luminous AGNs, showing all the above characteristics. They are easily recognized by their strong, broad permitted emission lines and their nonstellar continuum. The line widths, if interpreted as being due to Doppler motion, are typically 3000-5000 km s-1, with extreme cases of up to 40,000 km s-1 Full Width at Zero Intensity (FWZI). Seyfert 1s show also strong narrow (400-1000 km s-1) permitted and forbidden lines. This is not so common in bright quasars, where, in many cases, the narrow lines are weak or absent. Examples of a z = 2.051 quasar spectrum and a Seyfert 1 galaxy spectrum are shown in Fig.1. The shape of the nonstellar continuum in quasars and Seyfert 1 galaxies is different at different energies. Over a limited frequency range it can be described as a power-law in frequency,
where F
Figure 1. Spectra of three AGNs.
= C
-
is the
observed flux in erg s-1 cm-2
Hz-1. The spectral index,
, is about 1-1.2 between
0.8-1.5{mu}m, 0.3-0.7 between 0.2-0.6 {mu}m and 0.7 at hard X-ray
energies (2-10 keV). Observations of high redshift quasars show
considerable steepening of the continuum
(
2) below
1000 Å. This is likely to be due, at least in part, to the large number
of neutral hydrogen absorption systems on the line of sight. There is
some indirect evidence that the intrinsic continuum at those
frequencies is much harder, at least up to 3 or 4 Ryd. (chapter 10).
The nonstellar continuum must have a sharp decline in the far
ultraviolet, the part not accessible to direct observations. This is
inferred from the comparison of the ultraviolet and X-ray data. A
useful parameter, which has been used to describe the relative flux in
those wavelength bands, is
ox, which is the
spectral index
needed to connect the two continuum points at 2500 Å and 2 keV. Its
value ranges from about 1.2, for several Seyfert 1 galaxies, to 1.8 in
some bright quasars. There are indications that radio-loud AGNs have
smaller ox compared
with the radio-quiet ones, i.e. they emit
more 2 keV energy per given optical luminosity. Some powerful radio
galaxies show a typical Seyfert type 1 spectrum. These are sometimes
classified as a separate group under the name of Broad Line Radio
Galaxies (BLRGs). There are some spectral differences that justify,
perhaps, this type of classification, mainly to do with the line
profiles. Here they are simply recognized as radio-loud Seyfert 1s,
much like the distinction between radio-loud and radio-quiet quasars.
