Annu. Rev. Astron. Astrophys. 1984. 22: 319-58 Copyright © 1984 by Annual Reviews. All rights reserved

2. THE INCIDENCE OF EXTRAGALACTIC RADIO JETS

2.1. A List of Known Jets

Table 1 lists data on 125 radio sources known to us (in mid-August 1983) to have jets by our criteria. (We use H₀ = 100 km s^-1 Mpc^-1 and q₀ = 0.5.) Column 2 gives the identification - galaxy (G) or quasar (Q) - and its redshift. Columns 3 and 4 measure the observer's frame core and total monochromatic powers - log₁₀P_core⁵ at 5 GHz and log₁₀P_tot^1.4 at 1.4 GHz. (The flux densities S_core⁵ and S_tot^1.4 are those most often available.) We use "typical" values for variable cores. Some S_core⁵ and S_tot^1.4 values are estimated from neighboring frequencies, assuming ⁰ spectra for the cores and ^-0.7 for the total emission. Column 5 gives the projected length d_j of the brighter jet in kiloparsecs. Column 6 categorizes the jet sidedness (see Section 3.1). Columns 7 and 8 codify the origin of the data (see the footnotes below the table). We refer to VLBI data on jetlike extensions of the cores in sources with larger-scale jets whether or not the extensions separately meet criterion (1.) of Section 1.2. Table 2 lists 73 sources with features meeting only some of our criteria; modest increases in data quality could promote them to Table 1.

2.2. Occurrence Rates of Jets

The rates of occurrence of detectable radio jets (by our criteria) can be determined in several samples representing different extragalactic source types.

Twelve (55%) of the 22 radio galaxies in the re-revised 3C sample (140) - henceforth 3CR² - with 10°, b 10° and z 0.05 have definite jets, and two more (9%) have possible jets. We exclude from the sample the weak source 3C 231 = M82, whose emission comes mainly from a galactic disk. A fifteenth galaxy (3C 338) has structure resembling a jet except that it does not align with the radio core (51). Jets are thus detected in at least 55%, and perhaps 65%, of this sample, whose median P_tot^1.4 is 10^24.43 W Hz^-1. Jets were also found in 82% (9 of 11) of well-resolved sources in a complete sample of B2 radio galaxies with m_pg < 15.7 (83). R.A. Laing (private communication) finds definite jets in 13 sources (55%), and possible jets in 5 more (20%), in an unbiased sample of 24 E and SO galaxies with 0° < < 37°, m_pg 14.0, and S_tot^2.3 > 0.035 Jy.

Forty-two extended 3CR² galaxies or probable galaxies with z > 0.4 or V > 20 have been mapped at the VLA with good dynamic range (Table 1, ref. L4). Only two (5%) of these powerful radio galaxies (median P_tot^1.4 = 10^27.36 W Hz^-1) have continuous jets; one other has an elongated knot between its core and one lobe.

Twenty-two extended 3CR² QSRs have been mapped at the VLA with good dynamic range (Table 1, refs. L4 and W1). Ten (45%) of this group (whose median P_tot^1.4 is 10^27.43 W Hz^-1, similar to that of the distant 3CR² galaxies) have definite jets, and five more (23%) have structure resembling the brighter parts of jets. Ten (40%) of 24 extended QSRs in a 966-MHz survey have jets or structure resembling them (173), while five of the eight most extended sources in a complete sample of 4C QSRs have jets (271).

Jets are thus detected in 65 to 80% of weak radio galaxies, and in 40 to 70% of extended QSRs; but (with similar instrumental parameters) in only < 10% of distant galaxies similar to the QSRs in radio power (49, 139, 173). Among the extended 3CR² QSRs, the jet detection rate increases with the relative prominence f_C = S_core⁵ / S_tot^1.4 of the radio core, apparently regardless of redshift: all six QSRs with f_C > 0.03 (but only two of the six with f_C < 0.005) have jets or features resembling the brightest parts of jets. The lack of detectable jets in distant 3CR² galaxies may be related to their lower values of f_c - the median f_C in the distant 3CR² galaxy sample is only 0.0005. The two with detected jets are 3C 200, with f_C = 0.018 and weak emission lines, and 3C 341 (Figure 6), with f_C = 0.0005 and strong emission lines. The relations between jet, core, and emission-line fluxes of 3CR² galaxies need clarifying, but the multivariate (P_tot, P_core, P_optical) luminosity function for radio jets may contain clues to the physics of energy transport in these sources. We urge observers to publish integrated flux densities of jets and lobes separately, to allow study of this function.

Figure 6. VLA map of the jets and cocoon in the distant radio galaxy 3C 341 at 4.9 GHz. Note the narrowing of the cocoon to the right of the figure, and the irregular brightness distribution of the jets (data of A.H. Bridle and E.B. Fomalont, in preparation).

2.3. Jets in Weak Sources?

2.3.1   SPIRAL GALAXIES   Several Seyfert galaxies (for a review and references, see 282) with 10^21.5 < P_tot^1.4 < 10²³ W Hz^-1 have S-shaped kiloparsec-scale radio structures that may be low-thrust jets being bent and disrupted by the ram pressure of a rotating gas disk. If this is correct, the radio sources in Seyfert spirals may differ from those in ellipticals and QSRs mainly by (a) the smaller power output of the central "engine" and (b) the inability of their jets to escape from the dense, rotating interstellar medium (ISM) of a spiral disk. Interpreting the Seyfert sources as continuous jets (282) is not yet obligatory, however (264). Several edge-on spirals with unusually bright compact cores (121, and references therein) also have radio features extending from their nuclei apparently near the galactic minor axes. The features are as yet too poorly resolved to meet our criteria for jethood (Section 1.2), but if they do result from nuclear activity, they could be weak analogs of jets in elliptical galaxies.

2.3.2   GALACTIC JETS   The S-shaped structure of Sgr A West (36, 84) and its relation to the velocities of the [Ne II] lines in the region can be interpreted as the result of collimated outflow from the galactic center at ~ 300 km s^-1 (35, 84). The extent to which Sgr A West is a weak parsec-scale analog of more active galactic nuclei is unclear, however; the S-structure has a thermal spectrum (unlike extragalactic jets; Section 4.2), and models involving tidal distortion of infalling clouds also fit the data (84). The binary star SS 433 has the only astrophysical jet whose velocity (0.26c) and precessional geometry relative to us are unambiguous (for reviews and references, see 120, 154). Both the ~ 100 light-day scale of the known radio structure and its typical radio luminosity (P_tot^1.4 = 10^15.8 W Hz^-1) are much less than those of extragalactic jets, but SS 433 allows us to observe the evolution of synchrotron-emitting plasma in precessing supersonic jets directly. There is also evidence for bipolar, and perhaps well-collimated, outflows from star-forming regions in dense molecular clouds - see (55) and (232) for reviews and references. The ability to measure velocities, densities, and temperatures in and around these nearby flows may help to test models of extragalactic jet production and propagation (133, 207).