Astron. Astrophys. 332, 10-18 (1998)

2. The sample

We have used the images of sources classified as CSSs from the compilation by Dallacasa & Stanghellini (1990). From this, we have chosen a sub-sample using quite simple criteria. First, all the sources dominated by jet emission were selected. From this, a list of sources was chosen to include all the sources showing large bent jets on the sub-arcsecond scale. A bend in a jet is considered large when the jet major axis changes direction by an angle ().

Such a value was adopted to avoid ambiguities in selecting candidates from the published images. These are due to the uncertainty in determining the Position Angle (PA) of the jet major axis. We are also aware that the images were made from observations done using a variety of arrays and frequencies, some of which were inappropriate for our purpose. The selected sources are listed in Table 3 below. Observations of four of them by us are presented here.

2.1. The observations

The four sources (0548 165, 1741 279, 2033 187 and 2147 145) have been observed with VLBI at 1.6 GHz and 5 GHz using different arrays and recording systems. Calibration observations on largely-unresolved radio sources were made during each observing session along with the observations of the target source. Table 1 summarizes the observations. After the correlation process the bulk of the data reduction was done using the AIPS package. The sources have been imaged using both AIPS and DIFMAP (Shepherd et al. 1995).

Table 1. Observing information

The source parameters in Table 2 are as follows: - column 1: source name; column 2: observing frequency; columns 3,4,5: beam major axis, minor axis in mas and PA in degrees; column 6: r.m.s. noise in the map far from the source; column 7: component label; column 8,9,10: major axis, minor axis in mas and PA in deg of each component; column 11: component peak brightness in mJy/beam; column 12: component total flux density in mJy; column 12: component spectral index.

Table 2. Sources parameters

2.2. Description of sources

The four sources above plus the other sources listed in the following Table 3 are described briefly here:

3C43 (0127 233)

The VLA image from Pearson et al. (1985) shows a highly misaligned triple structure. The MERLIN map by Sanghera et al. (1995) shows that the northernmost component A in the EVN 18cm image by Spencer et al. (1991) is likely to be the core. This is consistent with the 50cm VLBI observations of Rendong et al. (1991). Spencer (1994) imaged 3C43 at 18cm with MERLIN. That image shows a bridge of emission between the main components and the northern component. The jet is straight and collimated for the first 150 mas, then it changes PA by and, after 75 mas the PA changes again by pointing towards east. According to Junor et al. (in preparation) the central component is 3.6% polarized at 8.4 GHz with the VLA. The polarized intensity is sufficiently strong to provide an estimate of Rotation Measure, RM, of rad m^-2 in the source's rest frame. The intrinsic magnetic field direction in the central component follows the curvature of the source faithfully.

3C99 (0358 004)

The radio source 3C99 has a triple structure on arcsecond scales. The outer components are located rather asymmetrically relative to the nucleus and have very different surface brightnesses (Mantovani et al. 1990). Along the major axis, 3C99 has an angular size of 6 arcsec and a linear size of 21 kpc. It is associated with an N galaxy which has been detected close to the central component (Spinrad et al. 1985). The central component is unpolarized in 8.4 GHz VLA observations (Mantovani et al. 1997) and it is likely to be the nucleus of 3C99. The VLBI image of the source at 18cm shows that the central component consists of several blobs of emission with the two prominent ones being significantly misaligned with the overall axis of the source.

3C119 (0429 415)

An image of the source structure with an angular resolution of 5 mas is presented by Nan Ren-dong et al. (1991). Component A has an inverted spectral index and it is identified with the core of 3C119. The morphology of 3C119 is rather peculiar. The jet emerging from the core component is not well collimated. It contains several blobs of emission. The major axis PA changes direction by to reach the component C, at about 40 mas from the core. From there, it changes direction several times to form an almost circular structure. The core has a radio luminosity which is of the source total luminosity at 5 GHz. Taylor et al. (1992) listed 3C119 among sources with very large Rotation Measures (RM 3400 rad m^-2). A number of possible explanations have been discussed in the paper by Nan Ren-dong et al. (1991) for the source's brightness distribution (e.g. rotational shear of the radio jet by an ambient rotating gaseous disk, precession in the nucleus, and the source expanding in a cavity in the interstellar medium), but those authors admit that they cannot reach a satisfactory conclusion.

3C147 (0538 498)

3C147 has been observed by several investigators; see, for example, the collection of images by Alef et al. (1990). Those authors have also observed 3C147 with 5 GHz VLBI over three epochs. The source shows an unusually-complex, nonlinear structure which varies with time. Superluminal separation of two components in the core region was observed also. New 8.4 GHz data (Alef, private communication) confirm a mildly-superluminal separation velocity of . The jet is embedded in a diffuse emission region and shows a change in the projected orientation of its major axis of at mas from the core. A VLA image at 1 GHz (van Bruegel et al. 1984) shows a weak component north to the main one in a position which is opposite to the jet respect to the core. 3C147 shows large RM's of -3144 and rad m^-2 in the rest frame of the source for the main component and for the extension to the NNE respectively (Junor et al. submitted).

0548 165

On arcsecond scales, the source shows an asymmetric structure with a strong, unresolved component coincident with a quasar at z=0.474 and a much weaker secondary component about 3 arcsec to the north. The faint, extended component is weakly polarized. Most of the polarization comes from the main component. This source has a RM of 1934 rad m^-2. The polarized emission is also strongly depolarized between 15 GHz and 5 GHz (Mantovani et al. 1994).

The mas scale structure of 0548 165 from a full-track, global 18cm program is shown in Fig. 1. The source has a straight jet pointing west which changes direction dramatically at 80 mas from the core.

Fig. 1. VLBI image of 0548 165 at 18 cm. Contours are -4, 4, 8, 16, 32, 64, 128, 256, 512 mJy/beam. The peak flux density is 692.6 mJy/beam.

The de-rotated magnetic field is aligned parallel to the east-west direction (Mantovani et al. 1994) like the straight part at the beginning of the jet.

EVN observations at 6cm (Fig. 2) confirm the main structure seen at 18cm. Fig. 2 also shows that the jet increases its width but it remains collimated and it seems to show a wiggling structure. The core is thought to be the easternmost component, since it has a flat spectrum. The detection of the secondary component about 3 arcsec north, suggests that either there is a counter jet or the VLBI jet bends back to the north.

Fig. 2. VLBI image of 0548 165 at 6 cm. Contours are -1, 1, 2, 4, 6, 8, 16, 32, 64 mJy/beam. The peak flux density is 110.9 mJy/beam.

3C287 (1328+254)

The image of 3C287 at 6cm with 7 mas resolution shows a regularly curving jet-like structure; this bears some similarity to that in 3C119 (Fanti et al. 1989). The source brightness decreases smoothly along the curved jet. It is not clear where the core is located so we cannot add its parameters to Table 3. Fanti et al. 1989 suggest that the main component (A in their Fig. 3) is the possible site for the core. This is the most compact feature visible at 6cm but it does have a spectral index of 0.5 between 18cm and 6cm (Nan Ren-dong et al. 1988).

Fig. 3. VLBI image of 1741 279 at 18 cm. Contours are -1, 1, 2, 4, 8, 16, 32, 64 mJy/beam. The peak flux density is 77.3 mJy/beam.

1442 101 (OQ172)

This object is unresolved by MERLIN. The VLBI image shows a very compact source mas in extent at 18cm (Dallacasa et al. 1995). It has a core-jet structure with the core located in the northern part of the radio emission. The source shows a bend in the jet major axis PA of at a separation of mas. 1442 101 has a redshift of 3.544 and a very high integrated RM of 22400 rad m^-2 in the source's rest frame (Taylor et al. 1992). Recent VLBA observations at 5 GHz by Udompresert et al. (1997) indicate that the RM is 40,000 rad m^-2 in the rest frame of the quasar. At 10 mas from the nucleus the RM falls to less than 100 rad m^-2. The very high RM is found near to the core; it is likely that this is not associated with material which could influence the bending of the jet.

1629 680

This source was observed at 13cm and 3.6cm (Dallacasa et al. 1997). The X-band image shows a straight jet mas long. The S-band image shows a mild bend in the jet which finally changes the direction of the axis by at 100 mas from the core.

1741 279

The VLA map at 8.4 GHz of 1741 279 shows two bright components roughly aligned E-W, a wiggling jet 5 arcsec long aligned N-S to the north of the two bright components and a region of weak emission 4 arcsec to the south-south-east. The two compact components have about 5% of the total polarized emission at 8.4 GHz (Mantovani et al. 1997). The magnetic field is parallel to the curved line joining the two components, changing direction smoothly by an angle of ; this suggests that there is a bend in the emitting region. The VLBI cm and cm maps, Figs. 3 and 4 respectively, show several knots along the axis between these two components. The eastern component is the nucleus (it has an inverted spectrum) and the western one shows an elongation north-south. When compared with the cm VLA image, it is possible to imagine that this elongation is the location of a sharp bend or cusp in the jet's apparent path.

Fig. 4. VLBI image of 1741 279 at 6 cm. Contours are -0.5, 0.5, 1, 2, 4, 6, 8, 16, 32, 64 mJy/beam. The peak flux density is 75.9 mJy/beam.

2033 187

This source is unresolved by the VLA A-array at 15 GHz (angular size arcsec) and is unpolarized. We present two VLBI images here. The first was obtained with a Global array at cm (Fig. 5); the second with the EVN at cm (Fig. 6). In these images, we see a straight jet and a dramatic bend at a small distance, 40 mas, from component `a'. Note that component `a' is the brightest feature in 2033 187 at both frequencies and shows a steep spectral index (). The position of the core is unknown. The resolution in the north-south direction is insufficient to resolve the westermost component at cm which has a low brightness region of emission extending south. The cm image shows a linear morphology from component `a' to `d', where the jet changes direction sharply.

Fig. 5. VLBI image of 2033 187 at 18 cm. Contours are -2, 2, 4, 8, 16, 32, 64, 128, 256 mJy/beam. The peak flux density is 303.2 mJy/beam.

Fig. 6. VLBI image of 2033 187 at 6 cm. Contours are -1, 1, 2, 4, 8, 16, 32, 64 mJy/beam. The peak The peak flux density is 94.8 mJy/beam.

2147 145

VLBI observations of 2147+145 were made for the first time by Cotton et al. (1984) at cm. That image shows a core-jet structure with the major axis PA of . Recent VLA images of 2147+145 made at 8.4 and 15 GHz, when compared with the previous observations by Cotton (1983), show that the total flux density at 15 GHz has increased by 13%. In addition, a new component is found north of the core in PA , separated by 0.35 arcsec (Mantovani et al. 1997). This component lies in a direction which differs by more than from that found for the jet in the cm VLBI map of Cotton et al. (1984). The VLBI source can be modelled by four Gaussian components that lie along a path which bends smoothly towards north. It is reasonable to expect that the jet continues with an increasingly pronounced bend to allow the flow to reach the component to the north.

Table 3. Source parameters.
Note -  References - column 2 -: a. Spencer at al. (1991); b. Mantovani et al. (1997); c. Nan Ren-dong et al. (1991); d. Alef et al. (1990); e. this paper; f. Fanti et al. (1989); g. Dallacasa et al. (1995); h. Dallacasa et al. (1997). - column 9 -: 1. Junor et al. (in preparation); 2. Mantovani et al. (1997); 3. Taylor et al. (1992); 4. Junor et al. (1997); 5. Mantovani et al. (1995).

The EVN observations of 2147+145 at 6 cm (Fig. 7) have produced an image that shows a core-jet structure which, at first sight, agrees with the 18cm image of Cotton (1984). A sharp bend in the jet occurs at 40 mas from the core. The flow changes direction by and a couple of weaker components are detected further north. The component `a', possibly the core, has a steep spectrum. The spectral index ranges from 0.38 (peak emission) to 0.7 (total emission) between 18cm and 6cm.

Fig. 7. VLBI image of 2147 145 at 6 cm. Contours are -0.6, 0.6, 1, 2, 4, 8, 16, 32, 64, 128, 256 mJy/beam. The peak flux density is 394.5 mJy/beam.

2.3. Summary of source parameters

Some of the parameters for the overall mas-scale structure that can be derived from the available images of these sources are summarized in Table 3. The table contents are as follows: - column 1: name; column 2: references for the images; column 3: separation (in milliarcsecond) between the core and the first large bend in the jet; column 4: linear separation, assuming km sec^-1 Mpc^-1 and ; column 5: difference in the jet PA before and after the bend (in degrees); column 6: redshift; column 7: optical identification; column 8: Rotation Measure in rad m^-2 corrected by the redshift. The symbol `-' means measurements are unavailable. The symbol `n' means polarization was not detected; column 9: references for RMs.

Because of the selection criteria adopted, the sources listed in Table 3 are characterized by the presence of bright jets. The change in the jet major axis Position Angle has a mean value of . The emission from the core compared to the total source emission is weak () in 3C43, 3C99, 3C119 and 3C147. However, it is strong () in 0548 165, 1629 680, 1741 279 and 2147 145. In two cases, namely 3C287 and 2033 187, it is still unclear where the core is located. It is worthwhile mentioning that all of the sources in Table 3 have been identified with quasars - with the exception of 3C99, which has been classified as an N-galaxy. Note, too, that the sources 2033 187 and 2147 145 are associated with unusually-faint optical objects (Cotton et al. 1989).

The sites where the bends occur are very close to the cores of the sources. The angular separation from the core to that location ranges between 15 and 300 mas, which corresponds to a linear separation of 40-900 pc, which is well inside the respective Narrow Line Regions (NLRs).

In general, most of the sources have a very high Rotation Measure. The sources are too compact to allow arcsecond-scale RM images. Those sources which are not polarized are also the most compact among those listed; this might be interpreted as due to large changes in the magnetic field direction within the angular resolution of the synthesized beam. In such cases, VLBI polarimetry is required; see, for example, the observations of 1442 101 (Udomprasert et al. 1997)

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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