5. CORRELATIONS BETWEEN OPTICAL SPECTRA AND RADIO PROPERTIES

The emission-line spectra of broad-line radio galaxies and Seyfert 1 galaxies are qualitatively quite similar, but there are a few characteristic differences between them. One is that the broad-line radio galaxies, in contrast to the Seyfert 1 galaxies generally have no detectable Fe II emission, or if present it is very weak. Another is that the Balmer decrement is generally steeper in the broad-line radio galaxies than in the Seyfert 1 galaxies. This is brought out in Fig. 8, a plot of the H / H ratio against Fe II/ H, in which it can be seen that the radio galaxies and Seyfert 1 galaxies are mostly well separated from one another.

Figure 8. Fe II / H emission-line intensity ratio versus H / H intensity ratio for Seyfert 1 galaxies (solid dots) and broad-line radio galaxies (open circles.)

This difference must contain some information about the physical difference between radio galaxies and Seyfert galaxies, but at the present time its interpretation is not known. It is interesting that the Fe II feature is also quite weak in most quasars, as shown by quantitative measurements by Phillips [36]. Although three quasars, 3C 273, PKS 0736+01 and 3C 48 were found by Wampler and Oke [33], Baldwin [37] and Boksenberg, Shortridge, Fosbury, Penston and Savage [38] respectively to have strong Fe II, Phillips has collected observations of 19 other quasars, only two of which show this feature up to a limit of one-tenth the strength of H, and therefore concluded that it is relatively rare in quasars.

Another difference between broad-line radio galaxies and Seyfert 1 galaxies is that in the former the [O III] lines are generally stronger with respect to H than in the latter. All of these properties must be seen as strong correlations, but not as absolute distinctions, for the Seyfert 1 galaxy IV Zw 29 has an optical spectrum indistinguishable from broad-line radio galaxies (see Fig. 9) yet according to the published literature is not known to be a radio source [19].

Figure 9. Measured spectrum of Seyfert 1 galaxy IV Zw 29 in relative energy units per unit wavelength interval versus wavelength. It appears very similar to the spectrum of a typical broad-line radio galaxy.

Although the division of Seyfert galaxies spectra into two two types on the basis of their spectra is a good one, inspection shows that there are intermediate objects having broad wings to the H I recombination lines but narrow central components, which might be described as intermediate-type Seyfert galaxies [39]. The spectrum of an extreme example, Mrk 609 is shown in Fig. 10. Its blue spectrum appears to be that of a typical Seyfert 2 galaxy, but in the red region the strong broad component of H can easily be seen. This and spectra of other intermediate-type Seyfert galaxies strongly suggest that there are two distinct physical regimes, the narrow-line region and the broad-line region, and that Seyfert 1 galaxies contain a relatively large portion of the broad-line region, while Seyfert 2 galaxies contain a relatively large portion of the narrow-line region, but that all intermediate cases between these two extremes can exist.

Figure 10. Measured spectrum of Seyfert 1.5 (intermediate type) galaxy Mrk 609, which appears in the blue spectral region to be a typical Seyfert 2, but has a broad component of H as seen in the red. The spectrum is plotted in relative energy units per unit wavelegth interval versus wavelength.

Finally let us discuss the correlation between radio properties of galaxies and their optical emission-line spectrum. Down to a given apparent magnitude a large fraction of the radio galaxies with emission lines are narrow-line radio galaxies, while only a minority are broad-line radio galaxies. For instance, in our early spectral survey made without any consideration of previous knowledge of line width or morphological type, eleven of the seventeen galaxies were narrow-line radio galaxies, five were broad-line galaxies, and one was the only intermediate-line radio galaxy we have found to date. This situation is in contrast to the Seyfert galaxies, where about two-thirds are Seyfert 1 (i.e. broad-line Seyfert galaxies) and only one-third are Seyfert 2 (narrow-line Seyfert galaxies). This of course might be a selection effect resulting from the way in which Seyfert galaxies are found optically. However, there may be more to it than this, for most of the Seyfert galaxies from which radio emission has been detected are Seyfert 2 galaxies. The data is given in Tables I and II where the "color-index" V-m_{6 cm} measures the strength of the total optical (V) luminosity of the galaxy with respect to the total radio (6 cm) luminosity of the galaxy. The index has been normalized so that for M 31 V-m_{6 cm} = - 3. Table I shows the ten radio galaxies with the largest V-m_{6 cm} from our first survey, 3C 405 = Cyg A having the strongest radio emission with respect to its optical emission. It can be seen that seven of these objects are narrow-line radio galaxies, while only three are broad-line radio galaxies. Table II gives the same information for the ten Seyfert galaxies with the brightest luminosity, taken from published work by de Bruyn and Willis [40] and by Kojoian, Sramek, Dickinson, Tovmassian and Purton [41]. It can be seen that the Seyfert galaxies are less luminous in the radio region than the radio galaxies, but note that the Seyfert galaxies with the greatest V-m_{6 cm} are only 1.5 to 2.5 magnitudes below the radio galaxy with the smallest V-m_{6 cm} index in Table I. The most interesting item in Table II is that nearly all the Seyfert galaxies with reported radio emission are Seyfert 2, or in two cases Seyfert 1.5, meaning that they have a strong admixture of the Seyfert 2 character in their emission-line spectra. It appears that the radio emission property is strongly correlated with the narrow-emission-line-emitting region in the ionized gas. These tables also illustrate the well known fact that most radio galaxies are N, cD, D or E galaxies, meaning that they have a high degree of symmetry about the center and little if any detectable interstellar matter, while the Seyfert galaxies in contrast are (in so far as they can be classified) mostly spiral galaxies, or related to spiral galaxies.