Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

2.3. FR I Radio Galaxies and Quasars

Systematic VLBI studies of FR I radio galaxy samples indicate that they are similar in morphology to FR II cores (Giovannini et al 1995, Pearson 1996): Asymmetric core-jet structures are typical (e.g. 3C 465, Venturi et al 1995) although in a few cases two-sided jets (e.g. 3C 338, Feretti et al 1993) or complex structures are found (e.g. 3C 272.1, Giovannini et al 1995). Jets tend to be well collimated close to the respective nucleus, they lie almost always on the same side as the larger-scale jets, and a higher degree of asymmetry seems to exist on small angular scales as compared with large angular scales. Stationary components are observed, as well as subluminal and slow superluminal motion (0.5-1.2 h^-1 c), which indicate that these jets are indeed relativistic. On the hypothesis that the radio structures are affected by Doppler favoritism, jet velocity and inclination to the line of sight can be derived from jet-to-counterjet ratios, from the ratio of core and total radio power, from Inverse-Compton arguments, and from imposing an upper limit to the deprojected size of a given source. Lorentz factors ~ 3 and viewing angles larger than 30° have been derived that are consistent with unification of low- and high-power radio galaxies (Giovannini et al 1994, 1995).

The best-studied objects in this category are the two nearby objects M 87 and Centaurus A. The radio galaxy M 87 (3C 274) contains the nearest powerful extragalactic jet in the northern hemisphere (Biretta 1996, Biretta & Junor 1995, and references therein) and offers unique opportunity for detailed study. This one-sided jet is characterized by filamentary features, limb-brightening, and side-to-side oscillations. The innermost part of the jet within 10¹⁶ cm from the center is well collimated and slightly curved (Junor & Biretta 1995). Prominent features on parsec and subparsec scales are stationary; at larger distances there is subluminal motion of _app = 0.28 ± 0.08 (Reid et al 1989); and on a kiloparsec scale, apparent superluminal motion is evidenced up to _app 2.5 (Biretta et al 1995). Together with the absence of a visible counterjet, these results are consistent with an underlying relativistic jet flow of Lorentz factor of about 2 and initial collimation at distances less than 0.1 h^-1 pc from a suspected black hole/accretion disk at the center (Biretta & Junor 1995).

The giant radio galaxy Centaurus A (NGC 5128) is the closest radio galaxy and contains a straight jet of about 50 milliarcsec in extent in the same direction as the arcsecond and X-ray jet in the source (Tingay et al 1994, Jauncey et al 1995). The source varies rapidly on time scales shorter than four months and shows subluminal motion of about 0.15 h^-1 c. The low speed and evidence for a subparsec-scale counterjet suggest that this jet is nonrelativistic or only mildly relativistic, that it is oriented at a large inclination, and that the innermost 0.4-0.8 h^-1 pc of the source is seen through a disk or torus of ionized gas that is opaque at lower frequencies owing to free-free absorption (Jones et al 1996).

The size of the compact radio nucleus of Centaurus A is 0.5 ± 0.1 milliarcsec (Kellermann et al 1997). The corresponding linear dimensions of 0.01 pc, about 10 light-days or 10¹⁶ cm, make this the smallest known extragalactic radio source. If the radio lobes are powered by a massive central engine, such as a black hole, their large total energy contents suggest that the central mass density may have exceeded 5 × 10¹³ M pc^-3, a value far larger than has been determined for any other AGN or quasar.

Two other prominent objects in this class are NGC 6251 (Jones & Wehrle 1994) and the core-dominated superluminal galaxy 3C 120, where the jet has been traced for about 500 milliarcsec from the core (Walker et al 1987, Benson et al 1988, RC Walker & JM Benson, personal communication).

A special case of a core-dominated source with FR I type radio morphology is 3C 84 (Figure 4), associated with the prominent Perseus cluster galaxy NGC 1275. On parsec scales, the source contains a broad complex jet with several components that exhibit subluminal motion of about 0.1 h^-1 c near the core and 0.5 h^-1 c at larger distances (see Romney et al 1995 and references therein; see also Venturi et al 1993, Krichbaum et al 1993b). A weak counter feature with strong low-frequency cutoff is found (Vermeulen et al 1994, Walker et al 1994); because of its low surface brightness, this cannot be explained by synchrotron self-absorption. Levinson et al (1995) have instead proposed obscuration through free-free absorption of the counterjet (but not the jet) by a torus or disk region ionized by the central continuum source. This raises concern about the validity of jet-counterjet ratio arguments made in explanation of source properties by beaming, although Walker et al (1994) conclude that the obscuring region is probably irrelevant at high frequencies and that many properties including the motion in this source are consistent with a mildly relativistic symmetric twin jet at modest inclination. The nucleus in 3C 84 shows radio emission out to a total length of 0.1 arcsec (24 h^-1 pc), which indicates activity of the central engine for centuries preceding the last outburst observed in 1960 (Taylor & Vermeulen 1996).

Figure 4. VLBA images 3C 84 at 15 and 43 GHz (from Dhawan, Kellermann, & Romney, in preparation). (left): 15 GHz; contours are drawn at -0.25, 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, and 95% of the peak, 3.49 Jy/beam. (right): 43 GHz; contours are drawn at -1, 1, 2, 4, 8, 16, 32, 64, and 95% of the peak, 0.75 Jy/beam.