Adapted from Kembavi & Narlikar (1999) ``Quasars and Active Galactic Nuclei'', Cambridge University Press, Secion 9.3
It was first noticed by B.L. Fanaroff and J.M. Riley (1974) that the relative positions of regions of high and low surface brightness in the lobes of extragalactic radio sources are correlated with their radio luminosity. This conclusion was based on a set of 57 radio galaxies and quasars, from the complete 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz into two or more components. Fanaroff and Riley divided this sample into two classes using the ratio RFR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. Sources with RFR < 0.5 were placed in Class I and sources with RFR > 0.5 in Class II. It was found that nearly all sources with luminosity
2 x
1025 h100-2 W Hz-1
str-1,
were of Class I while the brighter sources were nearly all of Class
II. The luminosity boundary between them is not very sharp, and there
is some overlap in the luminosities of sources classified as FR-I or
FR-II on the basis of their structures. For a spectral index of
1
the dividing luminosity at 5 GHz is
7 x
1023 h100-2 W Hz-1
str-1.
At high frequencies the luminosity overlap between the two classes can be as much as two orders of magnitude.
Various properties of sources in the two classes are different, which is indicative of a direct link between luminosity and the way in which energy is transported from the central region and converted to radio emission in the outer parts. We will now provide a somewhat more detailed description of the Fanaroff-Riley classes.
Fanaroff-Riley Class I (FR-I)
8 deg that varies along its
length. Along the jet the component of the magnetic field in the
plane of the sky is at first parallel to the jet axis, but soon
becomes aligned predominantly perpendicular to the axis (see Figure
9.10).
FR-I sources are associated with bright, large galaxies (D or cD) that have a flatter light distribution than an average elliptical galaxy and are often located in rich clusters with extreme X-ray emitting gas (Owen and Laing 1989, Prestage and Peacock 1988). As the galaxy moves through the cluster the gas can sweep back and distort the radio structure through ram pressure, which explains why narrow-angle-tail or wide-angle-tail sources, say, appear to be derived from the FR-I class of objects.
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Figure 9.3. VLA map of the FR-I galaxy 3C 449 at 1465 MHz, with angular resolution 4.8 x 3.4 arcsec2. The peak flux is 22.2 mJy per beam, with contours drawn at 5 per cent intervals, beginning with the -5 per cent contour. Reproduced from Perley, Willis and Scott (1979). |
A typical FR-I galaxy is shown in Figure 9.3 (Perley, Willis and Scott 1979). This is the radio source 3C 449, which is optically identified with a galaxy of type cDE4 at a redshift of 0.0181, so that 1 arcsec corresponds to 255 h100-1 pc. There are twin jets that are straight for ~ 30 arcsec from the core, after which they deviate towards the west and terminate into diffuse lobes. These jets and outer lobes are mirror symmetric about an axis through the core. The jets are generally smooth in appearance, but higher resolution observations show knots on a smooth ridge of emission, the southern jet being more knotty than the northern one. Within ~ 10 arcsec of the nucleus, the surface brightness of the jets is much reduced. The jets widen at a non-uniform rate close to the core, with the greatest expansion occurring where the jets are faintest. Beyond ~ 10 arcsec from the nucleus the opening angle is constant at ~ 7 deg. The emission from the jets is highly polarized, the average polarization over the jets being ~ 30 per cent, and the projected magnetic field is perpendicular to the jet axis.
Fanaroff-Riley Class II (FR-II)
FR-II sources are generally associated with galaxies that appear normal, except that they have nuclear and extended emission line regions. The galaxies are giant ellipticals, but not first-ranked cluster galaxies. Their average absolute magnitude < MR > = -19.9 (H0 = 100 km sec-1 Mpc-1) is close to the characteristic value M* of the Schechter galaxy luminosity function (see e.g. MB81, p. 352) at which the density of galaxies shows an exponential turnover. The environment of FR-II sources does not show enhanced galaxy clustering over the environment of randomly chosen elliptical galaxies (Owen and Laing 1989, Prestage and Peacock 1988). Owing to the large differences in the nature of the host galaxies and the environments of the FR-I and FR-II sources, it is possible that they are intrinsically different types of source not related to each other through an evolutionary sequence (but see the remark towards the end of this section).
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Figure 9.4. A VLA map of the FR-II quasar 3C 47 (Bridle et al. 1994) made at 4.9 GHz with 1.45 x 1.13 arcsec2 resolution. G is the core, A the jetted hotspot. H does not meet the hotspot criteria of Bridle et al. The figure was kindly provided by Alan Bridle. |
Host galaxies
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Figure 9.5. CCD image in the B filter of the MRC radio galaxy 1222-252 taken with the 1 m telescope of the Carnegie Observatory at Las Campanas. The image was kindly provided by A. Mahabal. |
Bivariate classification
Lop2. When the two-dimensional distribution
of points is projected
onto the radio luminosity axis, the two types remain separated but
with some mixing at the boundary. The dependence of the dividing radio
luminosity on the optical luminosity is a reflection of the form of
the bivariate luminosity function (see Section 7.11).
Physical distinction
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Figure 9.6. Distribution of FR-I and FR-II radio galaxies as a function of their 1400 MHz radio luminosity and absolute B magnitude. Reproduced from Owen and Ledlow (1994) in BDQ94. |
A model of the first kind has been proposed by De Young (1993), who has made an earlier version of Figure 9.6 as the basis. He notes that for a given radio luminosity in the figure, there is an optical luminosity limit that separates the two types of source. Sources fainter than this limit are FR-II, while those brighter than it are FR-I. Since the transition occurs at a fixed radio luminosity, De Young concludes that there is no major change in the engine that produces the radio emission, and the difference between the two types must be an environmental effect. (1)
De Young suggests that jets in FR-I galaxies are decelerated a short distance outside the production region. Thus, in the short distance before significant deceleration begins, the jets interact very little with the matter, and have low luminosity, which explains the gap often found between the nucleus and the base of jets in FR-I sources (see Section 9.4). After the deceleration the jets must proceed relatively unimpeded over large distances, as otherwise the outflow would cease completely. The deceleration can be produced by transfer of momentum from the jet to a dense ambient gas if the Reynolds number is very large, which is likely to be the case for reasonable jet dimensions and speeds. The dense ambient medium can be produced by the inflow of gas into the central region, owing to stellar mass loss, flows set up by interactions, cooling flows etc.
It is expected that active star formation will take place owing to the enhanced density of the central region, and also the action of the jet on it. This should make the central regions of FR-I galaxies bluer than those in FR-II galaxies. Testing such a prediction requires good signal-to-noise images in two or more colours of a number of radio galaxies, and a detailed study along these lines is yet to be made.
The second possibility, that the differences between the FR-I and FR-II galaxies are due to qualitative differences in the properties of the central engine, has been considered by Baum, Zirbel and O'Dea (1995). They base their conjecture on a detailed study of the correlations between radio luminosity, emission line luminosity and host galaxy magnitude of a large sample of FR-I and FR-II galaxies; this spans 10 orders of magnitude in luminosity and contains a number of galaxies of the two types that overlap in luminosity.
The principal differences found in the two types by Baum et al. are as follows.
The observed distribution of the [O III] /
H line ratio in FR-I
galaxies is similar to the distribution for radio-quiet ellipticals
and cooling flow galaxies. Baum et al. suggest that the emission lines
here are of the low ionization type, which are different from the high
ionization lines produced owing to ionizing radiation from the nucleus
in Seyfert and other active galaxies. Since the line luminosity is
also correlated with the host galaxy optical luminosity, Baum et
al. conclude that the line emission in FR-I galaxies could be produced
by processes in the host galaxy. In contrast to this, available
evidence suggests that in FR-II galaxies the lines are produced as a
result of an ionizing continuum from the nucleus. This points to a
possible important difference between the central engines of FR-I and
FR-II galaxies: the engines in the former produce far less ionizing
radiation and funnel a higher fraction of their total energy output
into the kinetic energy of the jets than FR-II galaxies.
Baum et al. furthermore suggest that FR-I sources are produced when the accretion rate onto the central black hole is low, and the black hole has relatively less angular momentum; the FR-II sources arise when the accretion rate is high and the hole spins more rapidly. The different degrees of black hole spin make a difference to the nature of the jets produced, leading to subsonic jets when the spin is low and supersonic jets when it is high. As we have mentioned above, these different jet properties can lead to different levels of interaction with the ambient medium, creating different radio morphologies. Baum et al. also suggest that a high accretion rate could decline with time, causing an FR-II galaxy to evolve into an FR-I type. The correlations that have been used in arriving at this picture, the conclusions drawn from them and the theoretical conjectures all need thorough study before the scenario can be accepted as plausible. We have seen above that the environments of the two types of radio galaxy are likely to be different. This seems to argue against the two types having an evolutionary connection. But it is possible that at least some of the FR-I galaxies began as FR-II galaxies in the dense environments of rich clusters and relatively quickly evolved to the FR-I state (Hill and Lilly 1991).