Proc. Natl. Acad. Sci. USA Vol. 92, pp. 11393-11398, December 1995 Colloquium Paper

This paper was presented at a colloquium entitled " and Active Galactic Nuclei: High Resolution Radio Imaging,." organized by a committee chaired by Marshall Cohen and Kenneth Kellermann, held March 24 and 25, 1995, at the National Academy of Sciences Beckman Center, Irvine, CA.

Possible links between BL Lacertae objects and quasars from very long baseline interferometry radio data D. C. GABUZDA* Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada

ABSTRACT Systematic differences in the very long baseline z = 0.68; 1823+568, z = 0.66) sources in this class; in other interferometry (VLBI) radio polarization structure and average words, in terms of their VLBI polarization structure, high VLBI component speeds of BL Lacertae objects and quasars BL Lacertae objects more closely resemble low red- support the view that the observational distinction between these shift BL Lacertae objects than they do quasars. classes, based in large part on the strength of their optical line Differences in Characteristic Superluminal Speeds. It has emission, is meaningful; in other words, this distinction reflects been suspected for several years (for example, see ref. 9) that significant differences in the physical conditions in these sources. the apparent speeds in the VLBI jets of BL Lacertae objects Possible models providing a link between the optical and VLBI are systematically lower than those in the VLBI jets of quasars. properties of BL Lacertae objects and quasars are discussed. It has recently become possible to show statistically that this is Most VLBI polarization observations to date have been global indeed the case, although there is clearly a fair bit of overlap observations made at 6 cm; recent results suggest that the VLBI between the two speed distributions; a Kolmogorov-Smirnov polarization structure of some sources may change dramatically test indicates that the BL Lacertae object and speed on scales smaller than those probed by these 6-cm observations. distributions are different with better than 99% confidence (2). Again, there does not appear to be a clear distinction between high and low redshift objects: if only quasars with I. Systematic Differences in the Very Long Baseline within the observed range for BL Lacertae objects for which Interferometry (VLBI) Properties of BL Lacertae speeds have been measured (z < 0.8) are considered, the Objects and Quasars difference between the BL Lacertae object and quasar speed histograms persists (also with >99% confidence). Differences in Polarization Structure. Our knowledge of the If it is assumed that the origin for the (usually) superluminal properties of the milliarcsecond (mas) scale jets in active motions seen on scales is relativistic motion in a jet galactic nuclei (AGN) has been greatly enhanced as the result oriented reasonably close to the line of sight, the apparent of polarization-sensitive VLBI observations, mostly made with speed of a component moving along the jet will be global arrays at 6 cm (see ref. 1-4 and references therein). BL Lacertae objects are highly variable, polarized AGN with ,B sin compact, flat-spectrum radio emission, a nonthermal contin- fCos4V' uum, and comparatively weak emission lines; they are usually 10ap Fanaroff-Riley type I (FRI) radio sources (5). Similar prop- where : is the intrinsic component speed and ' is the angle of erties are displayed by many core-dominated quasars, which the motion to the line of sight. For a given value of ,B, this have stronger emission lines and are FRII radio sources. Clear function is peaked at k = arccos3. It is clearly not trivial to differences have emerged between the polarization properties uncover the origin for the systematic difference in VLBI of the parsec-scale jets in quasars and in BL Lacertae objects: component speeds of BL Lacertae objects and quasars, since the inferred magnetic fields in quasar jets tend to be parallel the observed apparent speed depends on both angle to the line to the local jet direction, whereas those in BL Lacertae object of sight and intrinsic speed for the moving feature. The jets of jets tend to be orthogonal to it (Fig. 1). It is usually supposed BL Lacertae objects and highly variable core-dominated qua- that the longitudinal magnetic fields in quasars are due to sars are usually considered to have fairly small angles to the shear. Perhaps the most natural interpretation of the trans- line of sight to the earth. The idea that orientation of the VLBI verse magnetic field structure observed in BL Lacertae objects jets of BL Lacertae objects especially close to the line of sight is that the polarized jet components are relativistic shocks in (4 appreciably less than arccosl3) might be the origin for the which the transverse component of the magnetic field has been lower speeds observed in these sources is attractive in some enhanced by compression (6, 7). ways, since BL Lacertae objects are generally believed to be It has sometimes been suggested that the relevant distinction among the most highly beamed sources (10, 11). There is is not between BL Lacertae objects and quasars but rather considerable evidence, however, that the jets of BL Lacertae between low and high redshift objects (for example, see ref. 8). are not oriented close to the of The available VLBI objects especially line sight polarization information suggests quite compared to those in quasars. For example, an angle to the line strongly that this is not the case. The transverse magnetic fields of sight of 35-40° has been derived for the jet in BL Lac by characteristic of BL Lacertae objects have been observed in Hughes et al. (7) and Mutel et al. (12); Ghisellini et al. (13) the VLBI jets of both low redshift (e.g., BL Lacertae, z = 0.07; 0454+844, z = 0.11) and high redshift (e.g., 1803+784, Abbreviations: VLBI, very long baseline radio interferometry; AGN, active galactic nuclei; FR, Fanaroff-Riley; mas, milliarcsecond; pc, The publication costs of this article were defrayed in part by page charge parsec. payment. This article must therefore be hereby marked "advertisement" in *Present address: Astro Space Center, P.N. Lebedev Physical Institute, accordance with 18 U.S.C. §1734 solely to indicate this fact. Leninsky Prospekt 53, 117924, Moscow, Russia. 11393 Downloaded by guest on September 25, 2021 11394 Colloquium Paper: Gabuzda Proc. Natl. Acad. Sci. USA 92 (1995)

1823+568 Total Intensity 3C345 Total Intensity I Peak(mJy/beam)=672. Peak(mJy/beam)=6940. A

La 53 8

0 0 _Z o .0co C a -40 C, ._i0 a) -4. aL8) a) 00 a4

uI _

5 0 -5 Relative Right Ascension (mas) 0 -5 Relative Right Ascension (mas)

3C345 Complex Polarization Peak(mJy/beam)=167.

B >~. D

_ o F 6q 85 8 0

CJ 0 44 .I P:a)8ia 9

0

I .. .1. .. - I C 5 0 -5 5 0 -5 -10 Relative Right Ascension (mas)

FIG. 1. Typical VLBI polarization structure of BL Lacertae objects and quasars, indicated by 6-cm global observations. The BL Lacertae object 1823+568 at epoch 1989.29: total intensity (A) and linear polarization (B), with contours of polarized intensity and electric field vectors superimposed. Quasar 3C345 at epoch 1984.23: total intensity (C) and linear polarization (D), with contours of polarized intensity and electric field vectors superimposed. Assuming the jet components are optically thin, the inferred magnetic fields are perpendicular to the electric field vectors shown.

suggest that the jets in BL Lacertae objects as a class lie within and quasars is associated with systematic differences in the -30° to the line of sight, while those in quasars lie within -40° intrinsic speeds for the components; this possibility is discussed to the line of sight. There also does not appear to be evidence below. It should also be pointed out that if some of the for an anticorrelation between app and the core dominance components for which speeds have been measured are rela- ratio R, as might be expected if the jets in BL Lacertae objects tivistic shocks-as is almost certainly true-the measured were oriented especially close to the line of sight (2). speed is the pattern speed for the shock, which can in some Thus, it seems more likely that the observed systematic cases differ significantly from the underlying physical speed of difference in VLBI component speeds of BL Lacertae objects the flow (for example, see ref. 14). Thus, it is possible that the Downloaded by guest on September 25, 2021 Colloquium Paper: Gabuzda Proc. Natl. Acad. Sci. USA 92 (1995) 11395 systematic difference in the component speeds for BL Lacer- lead to systematically lower apparent VLBI component speeds tae objects and quasars is associated with different character- and a higher prevalence of transverse shocks in these sources. istic pattern speeds for these components compared to the The idea that jet speed and stability on VLBI scales could underlying flow. play an important role in the relationship between BL Lacer- tae objects and quasars has also been suggested by Duncan and II. Models Providing a Link Between BL Lacertae Hughes (18) on the basis of hydrodynamic simulations of And Quasars relativistic jets. Somewhat to their surprise, they found evi- dence that even within the relativistic regime, jets with com- By definition, the emission line equivalent widths of BL paratively low y (-5, for example) are considerably less stable Lacertae objects are small (15, 16). A better indicator of the than jets with comparatively high y ('10, for example) and "strength" of the line emission is probably the emission line suggested that the dominance of transverse shocks in the VLBI luminosity. The emission lines in BL Lacertae objects are jets of BL Lacertae objects but not in quasars could be nearly always an order of magnitude or more less luminous understood if BL Lacertae objects have intrinsically lower ry than those in quasars, although the emission line luminosities than quasars. In addition, there is some theoretical support for for a few BL Lacertae objects approach those for quasars (ref. a connection between low accretion rate and low jet speed; for 5 and references therein; C. Lawrence, personal communica- example, Blandford (19) has suggested that a low accretion tion). Thus, although the division in emission line luminosities rate around a slowly rotating black hole may give rise to jets separating BL Lacertae objects and quasars is not sharp, a that rapidly decelerate. systematic difference in emission line luminosity between the In such pictures, one would probably expect more or less a two classes does clearly exist. The fact that certain VLBI continuum of sources with differing accretion rates and emis- properties-polarization structure and apparent component sion line strengths; it is possible that the substantial minority speed-are systematically different for BL Lacertae objects of BL Lacertae objects that have FRII arcsecond-scale struc- and quasars offers further evidence that the optical observa- ture and/or luminosity (20) could be sources with "interme- tional distinction between these sources is not arbitrary but diate" accretion rates. If the accretion rates in BL Lacertae rather has some significant physical basis. objects are in fact systematically lower than those in quasars, It is useful to try to devise models in which it would be it is also possible that quasars could evolve into BL Lacertae natural for sources with weaker emission lines to have the objects if the accretion rate decreases in time, due, for observed VLBI properties of BL Lacertae objects. Two obvi- example, to a decreasing supply of gas to feed the black hole ous possible origins for the lower line luminosity in BL (see ref. 17). Lacertae objects are that these sources have a lower gas content or a less powerful ionizing UV continuum than do III. Recent Results quasars. Let us suppose that the origin of the weaker line emission in BL Lacertae objects is simply a smaller quantity of Possible Shocks in 3C345 and 3C454.3. Six-centimeter emission line gas and that the central engine powering the VLBI polarization images have shown the dominant magnetic AGN is a massive black hole (2). It seems plausible that a lower field in the jet of 3C345 to be longitudinal (21). Preliminary gas content in the host could lead to a lower accretion analysis of recent 2.8-cm images of this source (D.C.G. and A. rate onto the black hole; this could in turn lead to the ejection Mioduszewski, unpublished data; Fig. 2) suggests that the of relatively weak and turbulent jets of relativistic material. In polarization electric vectors in jet components roughly 1 mas this picture, quasars would have a higher gas content, higher from the core align with the local jet direction. Assuming these accretion rates, and correspondingly stronger, more stable jets. components are optically thin, the inferred magnetic field We might expect transverse shocks to form more easily in the would be transverse, indicating that the polarization from these comparatively less stable jets of BL Lacertae objects. In components may be dominated by transverse shocks. Similar addition, it would be natural for the component speeds in the results have been obtained in recent 1.3-cm polarization jets of these sources to be on average lower than those studies and spectral index studies conducted by Lepannen, observed in quasars, since the jets themselves would be less Lobanov, and collaborators using the very long baseline array powerful. (22). This suggests that although the effects of such transverse A similar picture may be derived from recent interpretation shocks are usually not apparent in the jet emission of quasars of the optical and radio observational differences between FRI on the scales probed by 6-cm global VLBI observations, they and FRII radio sources by Baum et al. (17). They point out that are more evident on smaller scales. the emission line luminosity for FRI sources is substantially The possibility that shocks can also form in the VLBI jets of less than for FRII sources of the same total radio luminosity quasars is supported as well by 6-cm global VLBI images of or same radio core powert and suggest that the collected 3C454.3 (23), which indicate that, although the dominant jet evidence indicates that this is associated with a lack of ionizing magnetic field is longitudinal, the polarization electric vectors UV continuum in FRI sources, rather than a lack of cold gas. in a recently emerged component align with the local jet They suggest that the most likely origin for the weaker UV direction, implying the local magnetic field to be transverse. In continuum in FRI sources is a lower accretion rate, possibly addition, although the dominant magnetic field in knots in coupled with a lower central black hole spin rate, compared to 6-cm images of 3C345 is longitudinal, the interknot emission FRII sources, and that one consequence of this might be lower appears to be more highly polarized than the knot emission, Mach numbers in FRI jets. If we suppose that BL Lacertae suggesting that the knots are weak shocks in which the degree objects and quasars should be primarily unified with FRI and of polarization has been decreased by partial cancellation of FRII sources, respectively, this picture suggests that the jets of the underlying longitudinal magnetic field and the transverse BL Lacertae objects should be slower than those in quasars; field due to compression (24). These results therefore imply the origin for the comparatively weak emission lines in BL that the longitudinal fields observed in the VLBI jets in quasars Lacertae objects would in this case be a weak UV continuum. at 6 cm do not necessarily indicate that shocks do not form in As above, the lower jet speeds in BL Lacertae objects would these jets, but rather that on the scales that have been sampled by the 6-cm observations the longitudinal field is sufficiently tThe observation that emission line luminosities in FRI sources are strong to dominate the transverse field in any shock compo- systematically lower than those in FRII sources provides intriguing nents. support for the unification of BL Lacertae objects with FRI sources The VLBI Polarization Structure of X-Ray BL Lacertae and quasars with FRII sources. Objects. In the past few years, increasing attention has been Downloaded by guest on September 25, 2021 11396 Colloquium Paper: GabuzdaPr Proc. Natl. Acad. Sci. USA 92 (1995)

3C345 Total Intensity 3C345 Complex Polarization I Peak(mJy/beam)=4020O P Peak(mJy/beam)-147. 'A. .B

cv -

10 0

0 -

0

a)

Q)

N

I.....1.1-11- ...... C? I -. 3 2 1 0 -1 -2 -3 2 1 0 1 -2 -3 -4

Relative Right Ascension (mas) Relative Right Ascension (mas)

FIG. 2. VLBI hybrid maps of the quasar 3C345 at 2.8 cm, epoch 1984.95: total intensity (A) and linear polarization (B), with contours of polarized intensity and electric field vectors superimposed. Assuming the jet components are optically thin, the inferred magnetic fields are perpendicular to the electric field vectors shown.

paid to the question of the relationship between BL Lacertae underlying magnetic field to be longitudinal, as is typical of objects detected through x-ray and radio surveys (XBLs and quasars rather than BL Lacertae objects. RBLs). Although XBLs tend to have weaker radio emission At first, it seems that the magnetic field structure observed than RBLs, a number of XBLs detected by the HEAO-1 Large in 1727 + 503 indicates that conditions in the VLBI jets of XBLs Area Sky Survey are being studied using polarization VLBI by and RBLs are quite different. However, there is a hint that the Kollgaard and collaborators. The first results from this effort magnetic field in 1727 +503 may change to being tranverse have recently been accepted for publication (25): mas-scale further down the jet (Fig. 3B); unfortunately, the dynamic polarization was detected in two of five x-ray BL Lacertae range of the polarization image is insufficient to allow this to objects. In both sources, polarization was detected in the VLBI be asserted with certainty. If this transition from longitudinal jet. In one, 1 133 + 703, the polarization electric vectors bear no to transverse magnetic field is present, however, it is reminis- obvious relation to the VLBI jet direction, but in the other, cent of behavior exhibited by the low redshift RBL 1219 + 285, 1727 + 503 (Fig. 3), the electric vectors in two knots in the inner in which the dominant jet magnetic field is transverse, but a jet are transverse to the local jet direction, indicating the new component has emerged with a clearly longitudinal mag-

1727+503 Total Intensity Complex Polarization I Peak(mJy/beamn)=63.3 Cntrs(%)= -1. 1. 1.4 2. 2.8 4. 5.6 8. 11. 16. 22. 32. 45. 64. 91. P Peak(mJy/beam)=4.97

A 0 -B

LO

0 0

C.) C) at) 0 aL) 0) aL)

ao Q) .I 0

uC)L I ~~.1 4\

5 0 -5 15 0 -5 -10

Relative Right Ascension (mas) Relative Right Ascension (mas)

FIG. 3. VLBI hybrid maps of the x-ray BL Lacertae object 1727+503 at 6 cm, epoch 1991.43: total intensity (A) and linear polarization (B), with contours of polarized intensity and electric field vectors superimposed. Assuming the jet components are optically thin, the inferred magnetic fields are perpendicular to the electric field vectors shown. Downloaded by guest on September 25, 2021 Colloquium Paper: Gabuzda Proc. Natl. Acad. Sci. USA 92 (1995) 11397 netic field (ref. 2; D.C.G. and T. Cawthorne, unpublished AGN, based in large part on the strength of their optical line data). emission, is a reflection of significant physical differences Evidence for a "Transition Zone" Several from the between these sources. If this is so, there may well be some Core? These results may point toward the existence of a reasonably direct connection between the optical and VLBI transition zone 2-4 parsecs (pc; 1 pc = 3.09 x 1016 m) in characteristics of these sources. projected distance from the core. In 3C345 and 3C454.3, the One possibility, for example, is that the accretion rates in BL jet polarization appears to be dominated by transverse shocks Lacertae objects are lower than in quasars, leading to the within -1 mas (-4 pc) of the core, but by a longitudinal field ejection of jets that are either intrinsically slow or rapidly further down the jet. Other evidence for substantially different decelerate and in which it is easy for transverse shocks to form. conditions in the small-scale jet of 3C345 is suggested by If the origin for the relatively weak emission lines in BL preliminary VLBI Faraday rotation maps (26, 27), which Lacertae objects is a relatively small mass of gas, this could suggest that there may be substantial Faraday rotation near the perhaps naturally lead to low accretion rates, providing a core but not further down the jet. One possible interpretation connection between the optical and VLBI properties of these of such a transition is that the magnetic fields in the jets in rates could lead to com- quasars are initially tangled but that over some distance the sources. Alternatively, low accretion longitudinal component to the field grows and eventually paratively weak UV continua, which would then provide the becomes predominant. reason for the weakness of the line emission in BL Lacertae There is also some evidence for the presence of a transition objects. In both of these pictures, the accretion rates in quasars zone in BL Lacertae objects, although this evidence is weaker. are higher, leading to the ejection of comparatively strong, In 1727+503 and 1219+285, the magnetic field within 2-3 mas stable jets in which the dominant magnetic field is longitudinal (2-3 pc) has a substantial longitudinal component, which due to shear. The apparent VLBI component speeds in BL appears to become weaker further down the jet. This possible Lacertae objects would naturally be lower than in quasars, transition appears to be at about the same projected distance since their jets would be slower. from the core as in quasars 3C345 and 3C454.3, but this could Higher resolution VLBI polarization observations are be a coincidence. One possible origin for such a transition from beginning to become available. Images made recently at 2.8 longitudinal to transverse jet magnetic field in BL Lacertae and 1.3 cm suggest that transverse shocks may be more objects in the context of the models discussed in Section II dominant in the jets of quasars nearer the active nucleus than above may be that, in association with a low accretion rate, the on the scales probed by 6-cm global VLBI observations. jets are initially comparatively fast but rapidly decelerate as There is also some evidence that, in some cases, longitudinal they propagate from the core. In this case, we might expect the magnetic fields are present in the jets of BL Lacertae objects magnetic field to initially be longitudinal due to shear where on smaller scales. Together, these results may be pointing the flow is rapid but switch to being transverse further from the toward the existence of a transition zone a few to a few tens core after the jet has decelerated and it becomes easier for of parsecs from the core. shocks to form. These results suggest that there may be a significant change in conditions a few mas from the cores of these sources, which I would like to thank my collaborators T. Cawthorne, R. Kollgaard, manifests itself in a fairly abrupt change in characteristic A. Mioduszewski, D. Roberts, and J. Wardle; John Wardle is due magnetic field structure. To uncover the physical origin for special thanks for a critical reading of this manuscript. I would also like to thank J. and B. for their interest and such a transition, it is necessary to estimate the actual linear Vetukhnovskaya Komberg Much of the work described here is currently being distance in the sources at which this transition occurs; this is encouragement. supported by an American Astronomical Society Henri Chretien difficult, since in the absence of knowledge of the angle of the International Research Grant. jets to the line of sight, we may infer only projected distance. Nonetheless, if we assume that the angles to the line of sight 1. D. T. D. H. & for the jets in these sources are 5-30°, the deprojected distance Gabuzda, C., Cawthorne, V., Roberts, Wardle, J. F. C. (1992) Astrophys. J. 388, 40-54. for an observed distance of -3 pc would be of the order of a 2. Gabuzda, D. C., Mullan, C. M., Cawthorne, T. V., Wardle, few to a few tens of parsecs. One obvious possibility is that this J. F. C. & Roberts, D. H. (1994) Astrophys. J. 435, 140-161. corresponds to a transition in the surrounding medium-from 3. Cawthorne, T. V., Wardle, J. F. C., Roberts, D. H., Gabuzda, narrow this the broad line to the line region, for example; D. C. & Brown, L. F. (1993) Astrophys. J. 416, 496-518. with the presence of substantial would perhaps be consistent 4. Cawthorne, T. V., Wardle, J. F. C., Roberts, D. H. & Gabuzda, Faraday rotation inside this zone but not outside it. It is of D. C. (1993) Astrophys. J. 416, 519-535. interest to determine whether there is evidence for a transition 5. Kollgaard, R. I. (1994) Vistas Astron. 38, 29-75. zone at a characteristic distance from the core in a larger 6. Laing, R. (1980) Mon. Not. R. Astron. Soc. 193, 439-449. number of sources and to determine whether the evidence for 7. Hughes, P. A., Aller, H. D. & Aller, M. F. (1989) Astrophys. J. such a zone exists for both BL Lacertae objects and quasars. 341, 68-79. Higher resolution VLBI polarization observations are clearly 8. Burbidge, G. & Hewitt, A. (1989) in BL Lac Objects, eds. needed to address these issues. Maraschi, L., Maccacaro, T. & Ulrich, M.-H. (Springer, Berlin), p. 412. IV. Summary 9. Mutel, R. L. (1990) in Parsec-Scale Radio Jets, eds. Zensus, J. A. & Pearson, T. J. (Cambridge Univ. Press, Cambridge, U.K.), p. The wealth of global 6-cm VLBI polarization data that is now 98. available has established that the characteristic magnetic field 10. Roberts, D. H., Gabuzda, D. C. & Wardle, J. F. C. (1987) Astro- structures in BL Lacertae objects and quasars on the size scales phys. J. 323, 536-542. probed by these observations are systematically different; the 11. Vermeulen, R. C. & Cohen, M. H. (1994) Astrophys. J. 430, inferred jet magnetic fields in BL Lacertae objects are trans- 467-494. verse to the local jet direction (a signature of relativistic 12. Mutel, R. L., Phillips, R. B., Bumei Su & Buccifero, R. R. (1990) shocks) while those in quasars are longitudinal (possibly a Astrophys. J. 352, 81-95. signature of shear). In addition, the apparent VLBI compo- 13. Ghisellini, G., Padovani, P., Celotti, A. & Maraschi, L. (1993) nent speeds observed in BL Lacertae objects are on average Astrophys. J. 407, 65-82. lower than those observed in quasars. These results indicate 14. Cawthorne, T. V. & Wardle, J. F. C. (1988) Astrophys. J. 332, that the observational distinction between these two types of 696-701. Downloaded by guest on September 25, 2021 11398 Colloquium Paper: Gabuzda Proc. Natl. Acad. Sci. USA 92 (1995)

15. Stickel, M., Padovani, P., Urry, C. M., Fried, J. W. & Kuhr, H. 22. Zensus, J. A., Krichbaum, T. P. & Lobanov, A. P. (1995) Proc. (1991) Astrophys. J. 374, 431-439. Natl. Acad. Sci. USA 92, 11348-11355. 16. Kuhr, H. & Schmidt, G. D. (1990) Astron. J. 99, 1-6. 23. Cawthorne, T. V. & Gabuzda, D. C. (1996) Mon. Not. R. Astron. 17. Baum, S. A., Zirbel, E. L. & O'Dea, C. P. (1995) Astrophys. J. in Soc., in press. press. 24. Wardle, J. F. C., Cawthorne, T. V., Roberts, D. H. & Brown, 18. Duncan, G. C. & Hughes, P. A. (1994) Astrophys. J. 436, L119- L. F. (1995) Astrophys. J. 437, 122-135. L122. 25. Kollgaard, R. I., Gabuzda, D. C. & Feigelson, E. D. (1996) 19. Blandford, R. D. (1994) in Proceedings of "The Physics ofActive Astrophys. J., in press. ," ASP Conference Series, eds. Bicknell, G. V., Dopita, 26. Wardle, J. F. C. & Roberts, D. H. (1994) in Compact Extraga- M. A. & Quinn, P. J. (ASP), Vol. 54, p. 23. lactic Radio Sources, eds. Zensus, J. A. & Kellermann, K. I. 20. Kollgaard, R. I., Wardle, J. F. C., Roberts, D. H. & Gabuzda, (National Radio Astronomy Observatory, Socorro, NM), p. D. C. (1992) Astron. J. 104, 1687-1705. 217. 21. Brown, L. F., Roberts, D. H. & Wardle, J. F. C. (1994)Astrophys. 27. Ochs, M. F. (1995) Ph.D. thesis (Brandeis University, Waltham, J. 437, 108-121. MA). Downloaded by guest on September 25, 2021