Annu. Rev. Astron. Astrophys. 1980. 18: 165-218
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4.3. Cores

4.3.1 TWO TYPES OF CORES Core emission in extended radio sources is of two types. First, there are the ultracompact flat-spectrum (alpha < -0.4) components with sizes 1 pc buried deep in the nuclei of the parent galaxies or QSOs. These components have similar properties to the (isolated) compact variable sources (Kellermann 1978). During the last decade compact cores have been shown to be present in extended radio sources of every morphological type. Flat-spectrum radio cores also frequently occur in the nuclei of elliptical and SO galaxies that are not extended radio sources (Ekers 1978, Crane 1979), and at a weaker level in some spirals (de Bruyn 1978). Even our own galaxy has a tiny flat-spectrum component near its center (Oort 1977 and references therein).

Second, about a quarter of the cores found in radio galaxies have steep spectra with indices smaller than -0.4 (Bridle & Fomalont 1978). These have typical sizes of a few kpc and are clearly different from the ultracompact cores. The best-studied examples are in galaxies - Virgo A/M 87 (Turland 1975a, Forster et al. 1978), Fornax A (Geldzahler & Fomalont 1978), and the giant edge-brightened double 3C236 (Fomalont & Miley 1975, Fomalont et al. 1979, Schilizzi et al. 1979). A steep-spectrum core has also been found in the quasar 3C207 (Joshi & Gopal-Krishna 1977). The steep-spectrum cores in Virgo A and 3C236 have been mapped in detail and in both cases they have basically double structure with compact flat-spectrum cores imbedded within them. The limited information available suggests that the spectral index distribution across steep-spectrum cores is remarkably constant (Berlin et al. 1975, Fomalont et al. 1979). Steep-spectrum cores having the steepest spectra tend to be the most luminous (Bridle & Fomalont 1978); a similar behavior is observed for the integrated luminosities and spectra of extended sources (e.g. Blumenthal & Miley 1979). Isolated steep-spectrum sources with sizes of a few kiloparsecs but no detectable extended emission frequently occur in spiral and Seyfert galaxies (Ekers 1978, de Bruyn & Wilson 1978). Despite the similarities it is not clear whether these isolated sources have any connection with the steep-spectrum kiloparsec cores in extended radio sources.

Since radio-core emission is almost certainly an indication of relatively recent nuclear activity, we may glean information about radio-source evolution by investigating possible relationships between the cores and the associated extended emission.

4.3.2 CORE LUMINOSITIES First we shall examine the relative strengths of cores in different types of extended radio sources. For many sources, measurements are of insufficient resolution to separate the flat- and steep- spectrum cores. Therefore, most studies have been made using combined core fluxes. Neither Fanti & Perola (1977) nor Bridle & Fomalont (1978) find evidence that the core luminosity, Pcore, increases with that of the extended emission, Pext, for radio galaxies, although Fanti & Perola cannot exclude a weak dependence.

Extended radio sources associated with QSOs tend to have relatively more powerful radio cores than those associated with galaxies (Ekers & Miley 1977, Riley & Jenkins 1977, Miley & Hartsuijker 1978, Owen et al. 1978b). Further data (G.K. Miley, T. Heckman, and R. Fanti, in preparation) indicates that extended quasars have cores that are on average about a factor of twenty stronger than radio galaxies having the same total luminosity. In addition, there appears to be a weak dependence of the core luminosity on the extended emission (Pcore propto Pext1/2) which would be consistent with the earlier work of Fanti & Perola (1977).

4.3.3 CORE STRUCTURES In several cases the core structures have been determined, primarily with VLBI techniques, and the morphologies of the cores have been related to those of the extended emission. This has been done for the flat-spectrum cores in Cygnus A (Kellermann et al. 1975), 3C 111 (Pauliny-Toth et al. 1976), 3C273 and 3C345 (Readhead et al. 1979), 3C390.3 (E. Preuss et al., in preparation), NGC 6251 (Readhead et al. 1978b, Cohen & Readhead 1979), Virgo A/M 87 (Kellermann et al. 1977 and references therein), and 3C 84/NGC 1275 (Pauliny-Toth et al. 1976), and for the steep-spectrum cores in 3C236 (Wilkinson 1972, Fomalont & Miley 1975, Fomalont et al. 1979, Schilizzii et al. 1979), Virgo A/M 87 (Turland 1975a, Forster et al. 1978), Centaurus A/NGC 5128 (Christiansen et al. 1977), and 3C 84/NGC 1275 (Miley & Perola 1975).

For the narrow sources whose extended emission is linear and symmetrically distributed (e.g. Cyg A, 3C111, 3C236, 3C390.3, NGC6251) the cores are observed to be aligned to within a few degrees of the outer lobes, even though the scales involved differ by as much as 5 × 106. Assuming the core emission to have been produced by relatively recent nuclear activity allows one to conclude that the orientation of the nuclear powerhouses must have remained fixed throughout the lifetimes of these sources. For the giant sources 3C236 and NGC 6251 the light travel time from their nuclei to their outer edges is ~ 107 yr implying a memory for direction in excess of this time. The only relevant direction that could feasibly remain fixed for such a long time is the angular momentum axis of a rotating compact object buried in the nucleus. Despite the overall agreement between the position angles of the core and extended structures in those sources there are fine-scale misalignments of a few degrees apparent both in the steep-spectrum core of 3C236 (Fomalont & Miley 1975, Schilizzi et al. 1979) and in the flat-spectrum core of NGC 6251 (Cohen & Readhead 1979). These may reflect bending in the cores similar to the wiggles observed in some radio jets (Section 4.4.2).

In contrast to the aligned cores in symmetric narrow sources, the four known cases of sources with one-sided extended emission (S or D2 sources in Section 2.1.1) have compact cores that are considerably bent (Readhead et al. 1978a). Two explanations have been proposed for this misalignment of cores in asymmetric sources. First, the cores may be bent by a pressure gradient in the nucleus. Second, a slight bending in the core might be apparently magnified if the radiating particles were moving at relativistic speeds along a narrow beam pointing approximately in our direction (Scheuer & Readhead 1979).

So far we have only considered the structure of cores in narrow sources. For the two wide doubles for which there is information, the cores and the extended structures are grossly misaligned, in Virgo A/M 87 by ~ 70° and in Centaurus A/NGC 5128 by ~ 45°. It is possible that the nuclear axes of the wide sources change appreciably during the source lifetime (Miley 1976). Alternatively there may be a shearing pressure gradient in the medium surrounding these galaxies which disrupts the beam and distorts the morphology of the outer extended radio emission. Such a process may well be occurring in 3C84/NGC 1275 which has a radio core that is aligned on scales of a parsec to 2 kpc but whose extended emission is completely amorphous. 3C84 is located at the center of the Perseus Cluster, which X-ray observations indicate contains some of the densest and hottest gas of any known cluster. Virgo A and Centaurus A also have strong X-ray halos.

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