Dual frequency VLBI polarimetric observations of 3C138

Transcription

1 Astron. Astrophys. 325, (1997) ASTRONOMY AND ASTROPHYSICS Dual frequency VLBI polarimetric observations of 3C138 W.D. Cotton 1, D. Dallacasa 2, C. Fanti 2,3, R. Fanti 2,3, A.R. Foley 4, R.T. Schilizzi 5,6, and R.E. Spencer 7 1 National Radio Astronomy Observatory, 52 Edgemont Road, Charlottesville, VA , USA 2 Istituto di Radioastronomia del CNR, Via P. Gobetti 11, I-4129 Bologna, Italy 3 Dipartimento di Fisica, Universitá di Bologna, via Irnerio 46, I-4126 Bologna, Italy 4 Netherlands Foundation for Research in Astronomy, Postbus 2, 799 AA Dwingeloo, The Netherlands 5 Joint Institute for VLBI in Europe, Postbus 2, 799 AA Dwingeloo, The Netherlands 6 Sterrenwacht Leiden, Postbus 9513, 23 RA Leiden, The Netherlands 7 NRAL-Jodrell Bank, University of Manchester, Macclesfield Cheshire, SK11 9DL, UK Received 16 December 1996 / Accepted 24 March 1997 Abstract. We report VLBI polarimetric observations of the steep spectrum compact radio source 3C138. The relatively small range of Faraday rotation measured in the outer jet and hotspot region suggests that the jet is not being confined by a dense plasma. These results are inconsistent with the frustrated jet scenario and are consistent with the young source scenario. Evidence is given for extensive plasma in the immediate region of the nucleus. The nuclear region is depolarized at 1.7 GHz and the high value of rad/m2 is measured for the rotation measure at a projected distance of 22/h pc from the nucleus. This suggests that the ionized gas giving rise to the broad emission lines may be confined to the region immediately surrounding the nucleus. Comparison with earlier results indicates that two knots in the inner jet are separating at an apparent rate of 6.3c/h. 1 Key words: techniques: polarimetric galaxies: quasars: individual: 3C138 galaxies: jets radio continuum: galaxies 1. Introduction Polarization sensitive images of radio sources from connected element interferometers have long been used to determine magnetic field structures in synchrotron sources and the presence of magnetized plasma inside and in front of the sources. In recent years, polarimetry using VLBI techniques has become increasingly common (Roberts et al. 1991; Cotton, 1993) but these observations are generally at a single frequency. Much of the Send offprint requests to: W. D. Cotton The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation. 1 H = 1h and q =.5 is used throughout this paper. understanding of the conditions in and around sources comes from comparing similar resolution polarization images at different frequencies in order to separate the effects of Faraday rotation and source magnetic field, depolarization, etc. One of the principle questions about Compact Steep Spectrum (CSS) sources is whether they are small because they are young (Fanti et al. 1995; Readhead, 1995) and therefore have not had time to expand to sizes of hundreds of kpc, or they are small because their jets have been bottled up by dense interstellar gas (van Breugel et al. 1984). Polarization sensitive measurements can address the question of dense gas stopping the jet. If such gas is present near the end of the jet where it impinges on the ISM and contains magnetic fields, then it should give rise to large Faraday rotation of the emission viewed through this magnetized plasma. The CSS radio source, 3C138 (= J ), is associated with a QSO with m v =18.84 and z=.759 (Hewitt & Burbidge, 1989). 3C138 is highly polarized ( 1%) and is used as a polarization calibration source for the Very Large Array (VLA). It has a very low integrated rotation measure of rad m 2 (Tabara & Inoue 198) and a rotation measure of is generally assumed for polarization calibration of the VLA. This quasar is currently quiescent as Netzer et al detected no optical variability. 3C138 was detected as a weakly variable source at 48 MHz by Fanti et al. (1981), but this is probably due to interstellar refractive scintillation rather than intrinsic variability of the source (Spangler et al., 1989). Gelderman & Whittle (1994) report strong, broad [O III] and Hβ lines which they interpret as a strong interaction between the jet and the interstellar medium. This source can be resolved by the VLA which was used for studies by van Breugel et al. (1984, 1992) at 15 GHz, Akujor et al. (1993) at 22 GHz and Akujor & Garrington (1995) at 8.4 and 15 GHz. This source consists of a core, a jet extending about 4 mas in position angle 65 and a low brightness counter jet extending 25 mas in the opposite direction.

2 494 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C138 4 Table GHz polarization calibrators Megawavelength 2-2 Source S VLA Poln VLA Poln VLBA Poln.Ang VLA Jy mjy mjy J J Notes: S VLA is the total intensity measured using the VLA, Poln VLA is the polarized intensity measured using the VLA, Poln VLBA is the polarized intensity measured using the VLBA, Poln.Ang VLA is the polarization angle of the E-vectors measured by the VLA Megawavelength Fig. 1. The uv coverage obtained for Global VLBI Network observations of 3C138 at 1.7 GHz. The Merlin array gives better separation of the different regions of 3C138. Total intensity images at 5 GHz are given by Fanti et al. (1989), Akujor et al. (1991) and Akujor et al. (1993). Previous total intensity VLBI images are given by Nan et al. (1991) at 61 MHz, Geldzahler et al. (1984) at 1.7 GHz and Fanti et al. (1989) at 5 GHz. Previous polarization sensitive VLBI observations at a single frequency (5 GHz) are presented in Dallacasa et al. (1995). If these observations are interpreted as having no Faraday rotation, then the magnetic field runs along the direction of the jet and then wraps around the end of the jet. In this paper we present new Very Long Baseline Array (VLBA) polarization measurements at 5 GHz and Global Network observations at 1.7 GHz. 2. Observations and data reduction GHz 3C138 was observed using the global VLBI network on 16 November 1992, using antennas at Effelsberg, Germany; Jodrell Bank, UK; Medicina, Italy; the Westerbork Array in the Netherlands; a single VLA antenna; and the VLBA antennas at Brewster, Washington; Ft. Davis Texas; Hancock, New Hampshire; Kitt Peak, Arizona; Los Alamos, New Mexico; North Liberty, Iowa; Owens Valley, California; Pie Town, New Mexico; and St. Croix, Virgin Islands. The data were recorded in both right and left circular polarizations in four 2 MHz bands centered at 1635, 1637, 1661, and 1663 MHz. The VLA and St. Croix failed to give results; only a single polarization was working at Hancock and the Medicina antenna apparently recorded no data on 3C138. The data were correlated using the VLBA correlator in Socorro, New Mexico in March The uv coverage obtained is shown in Fig. 1. Polarization calibration followed the general method of Cotton (1993); the instrumental polarization was determined using the sources J (=AO ), J (=DA 193) and J (=OJ287). The calibrators J and J were observed using the VLA on 18 November 1992 and total intensity and polarization values were determined from a calibration using (ironically) 3C138. These measurements are summarized in Table 1. The VLBI observations of 3C138 were interspersed with those of the calibrators J , J and J All post correlation processing was performed in the NRAO AIPS software package. The data were fringe fitted using the method of Schwab and Cotton (1983). Amplitude calibration initially used the measured system temperatures and assumed antenna sensitivities. The flux density scale was then adjusted constraining the flux densities of the calibrators to agree with the flux densities measured using the VLA. Due to the large size of 3C138 and the large spanned bandwidth, the different 2 MHz bands were not averaged together. Only phase self calibration was used in the imaging of 3C138. The final CLEAN recovered 5.37 Jy or 7% of the total flux density of 7.65 Jy. Since the Hancock antenna had only a single polarization recorded, a complex deconvolution of the polarized images was used (Cotton, 1993). Due to the low polarization of the calibrators and the large instrumental polarization of the antennas, the calibration of the position angle of the E vectors gave inconsistent results. However, the lack of significant rotation measure in 3C138 means that the integrated polarization angle is nearly the same at 1.7 and 5 GHz. The polarization of the 1.7 GHz image was adjusted such that the position angle derived from the sum of the Q and U CLEAN components agreed with the same measure derived from the better calibrated 5 GHz image discussed later. A comparison of the polarization vectors at and GHz shows no regions of high Faraday rotation. The total polarized emission represented in the derived images is 554 mjy or 86% of the total of 648 mjy measured with the VLA. In order to test for variable ionospheric Faraday rotation, the data were divided in half and imaged independently. The agreement of the derived linear polarization indicates that ionospheric Faraday rotation was not significant during these observations.

3 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C Megawavelength Megawavelength Final imaging used only phase self calibration and only the VLBA data were included. The total intensity represented in the image is 3.58 Jy or 96% of the total of 3.74 Jy; the integrated linear polarized intensity is.37 Jy. 3. Results GHz The full field of 3C138 is shown in Fig. 3 which includes the region of the counter jet, the detection of which is probable in this image. The core and jet region showing polarization vectors is given in Fig. 4; there was no detectable polarization in the region of the counter jet. The RMS noise in this image is 2.7 mjy/beam in total intensity and.6 mjy/beam in linear polarization. The fractional polarization after correction for the positive bias is shown in Fig. 5. Fig. 2. The uv coverage obtained from VLBA observations of 3C138 at 5 GHz. Table 2. VLBA 5 GHz polarization calibrators Source S VLA Poln VLA Poln VLBA Poln.Ang VLA Jy mjy mjy J J Notes: S VLA is the total intensity measured using the VLA, Poln VLA is the polarized intensity measured using the VLA, Poln VLBA is the polarized intensity measured using the VLBA, Poln.Ang VLA is the polarization angle of the E-vectors measured by the VLA GHz 3C138 was observed using the VLBA and a single antenna of the VLA on 19 December Two 8 MHz bands in each of right and left circular polarization were recorded centered at 4.7 and 5. GHz. Observations of 3C84 were used to determine the relative phases of the different frequencies and polarizations. The calibration followed a similar procedure to the one that was used for the 1.7 GHz data. Calibration was based on J and J which were observed with the VLA on 23 December 1994 using 3C48 for flux density calibration and 3C138 and J (=OQ28) for polarization calibration (see Table 2). J was used to calibrate to polarization angle of the VLBA data. The uv coverage obtained for 3C138 is shown in Fig. 2. The use of two widely spaced bands significantly improves the coverage, especially on the longer baselines. The use of multiple frequencies and the variable spectral index across the source reduces the dynamic range of the derived image. 3C138 was heavily resolved on all baselines used and in order to achieve a reasonable starting model, the Merlin and EVN measurements of Fanti el al. (1989) were initially included GHz The image of 3C138 at 5 GHz is given in Fig. 6; this image has been tapered to improve the sensitivity to the low surface brightness regions. The RMS noise in this image is 1. mjy/beam in total intensity and.2 mjy/beam in linear polarization. The fractional polarization after correction for the polarization amplitude bias is shown in Fig Comparison of 1.7 and 5 GHz results In order to compare the 1.7 and 5 GHz results, they need to be converted to the same resolution. Ideally, the uv coverages should be the same but this is not possible with VLBI arrays. The 1.7 and 5 GHz images were convolved to a common resolution and are given in Figs. 8 and 9. Absolute position information is lost through the use of self calibration techniques; the 1.7 and 5 GHz images were aligned relative to each other by forcing the position of peak brightness in the core to be coincident. Since all of the other structures in the source are much larger than the core, this procedure appears to be adequate. Many of the apparent differences in these two images are the result of the better sampling at 1.7 GHz of the region of the uv plane where the different components of the source become resolved. The spectral index image between 1.7 and 5 GHz derived from Figs. 8 and 9 is shown in Fig. 1. The core has a flat spectrum but the spectrum quickly steepens and is relatively constant over much of the jet. The apparent flattening of the spectrum along parts of the margin of the jet is undoubtedly an artifact of the different UV coverages used at the two frequencies; especially from the inadequate coverage at 5 GHz to image a source of this size and complexity. The rotation measure derived from these two images is shown in Fig. 11. Since there are only two frequencies involved in Fig. 11, there is ambiguity in rotation measure of 97 rad m 2. As the range of rotation measure in Fig. 11 is smaller than this ambiguity, and the integrated rotation measure is very close to, there should be no ambiguities in the rotation measures shown. The apparently anomalous values of the rotation measure in the

4 496 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C Fig. 3. The full field of 3C138 at 1.7 GHz rotated on the sky by 25 ; low surface brightness emission is visible in the counter jet. The resolution is 15 5 mas with position angle 12 and is shown in the lower left corner. The peak in the image is 241 mjy/beam and contours are drawn at -6, -4, 4, 6, 1, 14, 2, 4, 6, 1, 14, and 2 mjy/beam Fig. 4. The core and jet of 3C138 at 1.7 GHz. Rotation on the sky and resolution are the same as Fig. 3. The peak in the image is 241 mjy/beam and contours are drawn at -5, 5, 8, 14, 19, 27, 54, 81, 135, 189, and 27 mjy/beam. The lengths of the polarization vectors are proportional to the polarized intensity and have the orientations of the E-vectors. 2% 4% 6% Fig. 5. The fractional polarization image obtained for 3C138 at 1.7 GHz. Rotation on the sky and resolution are the same as Fig. 3. The wedge at the top shows the values of the fractional polarization.

5 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C Fig. 6. The image obtained for 3C138 at 5 GHz rotated on the sky by 25. The resolution is 5 mas and the beam is shown in the lower left corner. The peak in the image is 215 mjy/beam and contours are drawn at -2, 23571,23571and2mJy/beam. The lengths of the polarization vectors are proportional to the polarized intensity and have the orientations of the E-vectors. 2% 4% 6% Fig. 7. The fractional polarization image obtained for 3C138 at 5 GHz. as shown in Fig. 6. The wedge at the top shows the values of the fractional polarization Fig. 8. The image of the end of the jet of 3C138 at 1.7 GHz rotated on the sky by 25. The resolution is mas with position angle 12 and is shown in the lower left corner. The RMS noise in this image is 2.7 mjy/beam in total intensity and.6 mjy/beam in linear polarization. Contours are drawn at -5, 5, 8, 14, 19, 27, 54, and 81 mjy/beam. The lengths of the polarization vectors are proportional to the fractional polarization and have the orientations of the E-vectors. center of the jet at 15 mas in Fig. 11 is due to the low values of the polarization in this region probably resulting from the beam depolarization in an area with large variations in the polarization angle. Fig. 12 shows the depolarization between 5 and 1.7 GHz (the ratio of the 1.7 GHz fractional polarization to that at 5 GHz). The average depolarization in this image is 1.1 indicating no significant depolarization over most of the jet The core The full resolution image of the region of the core from the 5 GHz measurements is shown in Fig. 13. Baselines shorter than 3 million wavelengths were excluded from this image to reduce the effects of the extended emission. The upper limit on the polarization of the western knot at 5 GHz is.4%; the very weak polarization of this component and its location at the end

6 498 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C Fig. 9. The image of the end of the jet of 3C138 at 5 GHz rotated on the sky by 25 and convolved to the resolution of Fig. 8. The RMS noise in this image is 3.2 mjy in total intensity and.6 mjy in linear polarization. Contours are drawn at -1, -6, 6, 1, 16, 22, 32, 64, 96 and 16 mjy/beam. The lengths of the polarization vectors are proportional to the fractional polarization and have the orientations of the E-vectors Fig. 1. The spectral index between 1.7 and 5 GHz derived from the images displayed in Figs. 8 and 9. The wedge at the top shows the corresponding gray scales for values of the spectral index between -1 and Fig. 11. The rotation measure derived from the images displayed in Figs. 8 and 9. The wedge at the top shows the corresponding gray scales for values of the rotation measure between -3 and 1 rad/m 2. The resolution is as in Fig. 1.

7 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C Fig. 12. The depolarization from 5 to 1.7 GHz derived from the images displayed in Figs. 8 and 9. The wedge at the top shows the corresponding gray scales for values of the ratio of the 1.7 GHz fractional polarization to the 5 GHz fractional polarization. 38 Table 3. Separation of knots at 5 GHz Epoch yr. Separation Reference mas Fanti et al. (1989) Dallacasa et al. (1995) this paper Fig. 13. The full resolution image of the core of 3C138 at 5 GHz in December 1994 rotated on the sky by 25. The polarization has been corrected for a rotation measure of rad/m 2 and the E-vector shown is the equivalent at infinite frequency. The resolution is mas and is illustrated in the lower right corner. The RMS off source noise in total intensity is.4 mjy/beam and in linear polarization is.15 mjy/beam. The peak in the image is 173 mjy/beam and contours areat-1,123,5,7,1,2,3,5,7and1mjy/beam. of the inner jet suggest that it is the location of the nuclear core. This conclusion is supported by Fig. 1. While the 1.7 GHz image has too low a resolution to clearly separate the two knots, the spectral index of the core region is flatter on the western end. This conclusion differs from that of Fanti et al. (1989) who concluded on the basis of apparent size that the eastern knot was the core. The first knot in the jet was detected in polarized emission at 5 GHz. Although the 1.7 GHz data does not have the resolution to separate the various components seen in Fig. 13. it shows no detectable polarized emission in this region. This suggests Fara- day depolarization of the region. Since the 5 GHz observations were made in two 8 MHz bands it is possible to look for large rotation measures by imaging the 4.7 and 5. GHz data separately. The rotation measure derived from a comparison of these images at the location of peak total intensity in the first knot is rad/m2 and a peak polarized intensity of 3.5%. The inferred intrinsic polarization angle (extrapolated to infinite frequency) is With only two measured frequencies, the determination of the rotation measure is ambiguous by multiples of 6622 rad/m 2. In order to resolve this ambiguity, the two adjacent 4 MHz wide bands at 4.7 GHz were imaged and compared. The derived rotation measure from these closely spaced frequencies was rad/m2 suggesting that there are no additional turns in the polarization angle between 4.7 and 5. GHz. Fig. 13 has had the effects of Faraday rotation removed and shows the magnetic field oriented parallel to the jet. The apparent separation of the knots in the core of 3C138 as a function of time is summarized in Table 3 and is shown in Fig. 14. A comparison of these values leads to an average rate of separation of.27 mas/yr or an apparent proper motion of 6.3c/h where H o = 1h and q o =.5. The scatter among the limited measurements suggests that this value is accurate to about 5%.

8 5 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C138 Table 4. Source parameters Area I 1.7GHz Q 1.7GHz U 1.7GHz P 1.7GHz Φ 1.7GHz I 5GHz Q 5GHz U 5GHz P 5GHz Φ 5GHz sp. index RM Depol. Jy Jy Jy Jy Jy Jy Jy Jy rad/m 2 outer jet inner jet <.2 core <.1 Notes: I 1.7GHz, Q 1.7GHz, U 1.7GHz are the integrated I, Q and U polarization at 1.7 GHz of the area; P 1.7GHz, Φ 1.7GHz are the polarized amplitude and orientation angle at 1.7 GHz; I 5GHz,Q 5GHz,U 5GHz are the integrated I, Q and U polarization at 5 GHz; P 5GHz, Φ 5GHz are the polarized amplitude and orientation angle at 5 GHz; sp. index, RM, Depol. are the spectral index, Rotation measure and depolarization ratio determined between 1.7 and 5 GHz. Separation (mas) Epoch Fig. 14. The apparent separation of the knots in the core of 3C138 at 5 GHz as given in Table Brightness asymmetries in the inner jet If the core is the westernmost knot visible in the jet then the inner jet is quite asymmetric as seen in both Figs. 4 and 13. The brightest feature in the jet is the eastern knot in Fig. 13, which is about 23 times the noise, corresponding to a sidedness ratio in excess of 23. The fainter knot at the extreme eastern end of the jet seen in the high resolution image is 2 times the noise. The inner jet can be seen on a slightly larger scale at 1.7 GHz. In Fig. 4, the inner jet can be followed up to about 35 mas from the core where it disappears. In this region the brightness is generally 15 times the noise. There is no reason to assume that there is a feature on the counter jet side comparable to the eastern bright knot so a conservative lower limit to the jet/counter jet brightness ratio in the inner jet is about Source parameters Table 4 gives the averaged source parameters measured in various regions of the source. The outer jet includes the region to the east of -5 mas in Fig. 4. The inner jet is the visible portion of the jet just east of the core in Fig. 4. The core includes the region shown in Fig. 13. The I, Q, and U values are integrals over the relevant portion of the image normalized by the beam area. The rotation measure could not be determined for the inner jet and core due to the lack of detection of polarized emission in this region at 1.7 GHz; only limits could be given for the depolarization. 4. Discussion 4.1. Evidence for ionized gas The optical spectrum of 3C138 as given by Gelderman & Whittle (1994) indicates a large amount of ionized gas in this object. The limited spatial resolution of this data cannot establish where in the source this material resides. Gelderman & Whittle interpreted their spectra as an indication of a strong interaction between the jet and the interstellar medium. The lack of high Faraday rotation or depolarization in the outer regions of the jet and the hotspots shown in Fig. 11 rule out this region as the location of this plasma. There is very low polarization in the core region as evidenced by Figs. 5 and 7 as well as in VLA observations by van Breugel et al. (1984), Akujor & Garrington (1995) and Akujor et al. (1993). The measurements of van Breugel et al. give 7% polarization of the core region at 15 GHz. The comparison of the 4.7 and 5. GHz data described above indicates the very large rotation measure of rad/m 2 at a projected distance of 22/h pc from the nucleus. Thus, most of the properties of the radio emission which could be due to intervening plasma appear confined to the region immediately surrounding the nucleus. If the strong optical line emission reported by Gelderman & Whittle is coming from this plasma, it is confined to the immediate region of the nucleus as well Relativistic motion The apparent superluminal separation of the knots in the inner jet shown in Table 3 and Fig. 14 is consistent with the general model of AGN radio sources in which the jet is assumed to be initially highly relativistic. The minimum Lorentz factor to account for the observed speed is γ = 6.3/h and the maximum angle between the jet direction and the line of sight is 18. With these values, there is no problem in justifying the brightness asymmetry of the jet. 3C138 is an X-ray source, with a flux of erg cm 2 sec 1 as measured by Rosat in the Kev band (Brinkman et al., 1995). We have computed the X-ray flux

9 W.D. Cotton et al.: Dual frequency VLBI polarimetric observations of 3C expected from inverse Compton scattering on the basis of the component fluxes and size, and compared it with the Rosat flux in order to get information on the Doppler factor, D. Component flux densities and sizes were determined by a least squares fitting of Gaussian components to the image. Following the method of Marscher (1983), the fitted FWHMs of the components were multiplied by a factor of 1.8 to convert the Gaussian size to an equivalent ellipsoid with uniform emissivity. Unfortunately, the limit we get, D.5, is not very useful to further constrain the orientation of the jet or the Lorentz factor Is 3C138 different from other CSS sources? The images of 3C138 presented here show a relatively undisturbed jet with a few bends and wiggles but without the sharp turns and distortions seen in many other CSS sources, e.g. 3C43 (Spencer et al., 1991); 3C48 (Wilkinson et al., 1991); and 3C38 (Akujor et al., 1991). These bends may result from interactions of the jet with the ISM. The lack of these strong distortions of the jet can be taken as additional evidence of the lack of a strong interaction of the jet in 3C138 with the local ISM; at least in the outer regions of the source. 3C138 is unusual among CSS sources in that it is very polarized, has very low rotation measure and little depolarization. These features suggest that there is much less intervening magnetized plasma than is typical in CSS sources. Brinkmann, W., Siebert, J., Reich, W. et al., 1995, A&AS, 19, 147. Cotton, W. D., 1993, AJ, 16, Dallacasa, D., Cotton, W. D., Fanti, C., et al., 1995, A&A, 299, 671. Fanti, R., Fanti, C., Dallacasa, D., et al., 1995, A&A, 32, 317. Fanti, R., Fanti, C., Ficarra, A., et al., 1981, A&AS, 45, 61. Fanti, R., Fanti, C., Parma, P. et al., 1989, A&A, 217, 44. Gelderman, R., Whittle, M., 1994, ApJS, 91, 491. Geldzahler, B. J., Fanti, C., Fanti, R., et al., 1984, A&A, 131, 232. Hewitt, A., Burbridge, G., 1989, ApJS, 69, 1. Marscher, A. P., 1983, ApJ, 264, 296. Nan, R., Fanti, R., Fanti, C., Schilizzi, R. T., 1991, A&A, 252, 513. Netzer, H., Heller, A, Loinger, F., et al., 1996, MNRAS, 279, 429. Readhead, A. C. S., 1995, Proc. Natl. Acad. Sci. USA, 92, Roberts, D. H., Brown, L. F., Wardle, J. F. C., In Radio Interferometry: Theory, Techniques, and Applications, T. J. Cornwell, R. A. Perley (eds.), A.S.P. Conf. Ser., vol 19, p Spangler, S., Fanti, R, Gregorini, L., Padrielli, L., 1989, A&A, 29, 315. Schwab, F. R., Cotton, W. D., 1983, AJ, 88, 688. Spencer, R. E., Schilizzi, R. T., Fanti, C., Fanti, F., et al., 1991, MN- RAS, 25, 225. Tabara, H., Inoue, M., 198, A&AS, 39, 379. Wilkinson, P. N., Tzioumis, A. K.,Benson, et al., 1991, Nature, 352, Conclusions Multi-frequency polarimetric VLBI observations of the compact steep spectrum quasar 3C138 give no evidence for a great deal of magnetized plasma in front of the end of the jet. This and the lack of serious distortions of the jet suggest that in this source the jet is not being frustrated by a dense interstellar medium. The depolarization and high values of the rotation measure seen in the region of the nucleus lead us to conclude that the plasma giving rise to the optical emission is likely to be found in the inner regions of the quasar. As the effects extend at least several tens of parsecs, this plasma must be from a region larger than that of the Broad Line Region. Apparent superluminal motion in the inner portion of the jet is consistent with the observed large jet/counter jet brightness ratio being due to Doppler boosting in a relativistic jet. The lack of counter hotspots suggests that much of the hotspot emission is also Doppler beamed. These results are consistent with the young source scenario in which 3C138 is a relatively young source which will eventually expand from its present kiloparsec size to hundreds or thousands of kiloparsecs. References Akujor, C. E., Garrington, S. T., 1995, A&AS, 112, 235. Akujor, C. E., Spencer, R. E., Zhang, F. J. et al., 1991, A&A, 25, 215. Akujor, C. E., Spencer, R. E., Zhang, F. J. et al., 1993 A&A, 274, 752. van Breugel, W. J. M., Fanti, C., Fanti, R., et al., 1992, A&A, 256, 56. van Breugel, W., Miley, G., Heckman, T., 1984, AJ, This article was processed by the author using Springer-Verlag LaT E X A&A style file L-AA version 3.