In this section we briefly describe the radio structure and the integrated radio spectrum of each source. The spectra in Fig. 1 (click here) have been obtained from measurements found in the literature. We used the proper error on the flux density when reported, otherwise we attributed a 5 formal error to the measurements. Images are presented in Fig. 2 (click here) for the sources with significant VLBI structure only, and the components we revealed here are labelled on the images accordingly with the tables described in Sect. 5.
Figure 1: Radio spectra of the CSS-GPS candidates observed here. The spectral points have been taken from literature
We usually compare the total flux accounted for by either our data or our image to the "average'' flux density at the two observing frequencies as derived in the literature, and we use FS and FX to indicate the fraction of flux density in the images at S and X bands respectively. Often we found discrepant measurements of the total flux density, particularly at high frequencies, indication of either large unquoted systematic errors (or confusion) or intrinsic source variability, most effective at X band. The comparison therefore should be taken with care and the fraction of the flux density reported here is only indicative of the possible presence of significant radio emission missed by the VLBI observations. We would like to remark that snapshot data taken with a limited number of antennas like in our case are likely to miss complex radio emitting regions with angular sizes larger than a few tens of milliarcsecond.
0201+113 [Q, z = 3.61]
Our images at both bands show a single component. The correlated flux density at X band (where the whole flux density is accounted for in our data) is larger than that at S-band (). Hodges et al. (1984) detected an unresolved component in their VLBI data at 18 cm.
The radio spectrum from nearly simultaneous observations obtained with the Very Large Array (VLA) is presented by O'Dea et al. (1990) and can be considered GPS-like with a peak around 4 GHz. However, some variability of the flux density has been observed at 5 GHz, and the value of 0.6 Jy at 43 GHz reported in the new VLA calibrators manual suggests a flattening of the spectrum at high frequencies and possibly confirms the variability of the flux density. The optical spectrum shows a damped Ly absorption feature at z=3.387 (White et al. 1993), confirmed by redshifted HI absorption against the quasar detected by de Bruyn et al. (1996).
0237-027 [Q, z = 1.116]
The X band image is dominated by a pointlike component responsible for the whole flux density, while in the S band the source is much weaker and marginally resolved in PA (). The spectrum built with data from literature is grossly of GPS type and peaks at about 5 GHz. However, a recent measurement at 5 GHz by Griffith et al. (1995) revealed a decrease of about 40% of the flux density with respect to other values previously published (e.g. Perley 1982), and other measurements at high frequencies suggest a possible flattening and/or radio variability of the source. The 90 GHz flux density reported by Steppe et al. (1992) is suggestive of either a rising mm component or significant variability at mm wavelengths.
This high redshift quasar was detected in the X-ray by the Einstein satellite (Wilkes et al. 1994).
0237-233 [Q, z = 2.223]
This high redshift quasar has a large number of absorption features in the optical spectrum (see for example Aldcroft et al. 1994). There is also an indication of X-ray variability (Wilkes et al. 1994).
The integrated radio spectrum reported by Kühr et al. (1981), O'Dea et al. (1990) and by Steppe et al. (1995) is typical of a GPS radio source with a spectral peak around 1 GHz. However by comparing the forementioned spectra with the flux densities reported in the new Parkes catalogue, we can infer a significant decrease of the intensity at all the frequencies, although the spectrum still is of GPS type (see Fig. 1 (click here)). On the contrary, 10 years of monitoring (Waltman et al. 1991) show a nearly constant flux density at both 2695 and 8085 MHz. The VLBI image at X band in Fig. 2 (click here) shows two very asymmetric components (flux density ratio 12 : 1) separated by about 12 mas and accounting for nearly the whole flux density at this frequency. The source is not resolved at S band (), due to the lower resolution. The S-band model and image presented by Preston et al. (1989) show an asymmetric double source elongated in PA 90 with a separation of about 20 mas between the two components.
0500+019 [G, z = 0.583]
There has been some debate on the optical identification: formerly associated with a quasar by Fugmann & Meisenheimer (1988), now the radio source host has been classified as a galaxy at z=0.583 by de Vries et al. (1995).
The source was not resolved with VLBI by Hodges et al. (1984) at 18 cm. Whilst the structure is barely resolved at S band (with ), at X band (Fig. 2) the radio emission shows two very asymmetric regions separated by 10 mas in PA and accounting for almost the whole flux density. Our data are consistent with a global VLBI image at 5 GHz by Stanghellini et al. (1997). The integrated radio spectrum peaks around 3 GHz and is rather flat at dm wavelengths and is similar to that presented by O'Dea et al. (1990). There is a significant steepening in the high frequency end ( 0.9 between 10.7 and 31 GHz). Millimeter observations reported by Wiren et al. (1992) are suggestive of an increased flux density at high frequencies, with a possible flattening of the spectrum.
0511-220 [Q, z = 1.296]
This quasar has a GPS type radio spectrum peaking around 5 GHz, although the high frequency spectral index is just about 0.5. Some variability can be inferred at both 1.4 and 8.4 GHz, although the amount of data is too limited to draw firm conclusions.
The VLBI structure is resolved at X band (see Fig. 2 (click here)) in PA and can be either a core-jet or an asymmetric triple (). The S band image instead shows unresolved emission with only, corresponding to about one third of the total flux density detected at X-band. Therefore the radio emission detected here has an inverted spectral index. However, the uncertainties in the accounted flux and structures are larger than in other sources since the source declination is rather low.
Figure 2: Images for the sources showing structure resolved in two or more components. The restoring beam for each image is reported in Table 2 (click here). The contour levels are -1, 1, 2, 4, 8, 16, 32, 64, 125, 250 times the first contour reported in Tables 3 (click here) and 4 (click here) for the X and S band images respectively
0743-006 [Q, z = 0.994]
This GPS quasar has not been resolved by the present observations. Within the errors, at X band the whole flux density of the radio source is accounted for in our image, while at S band the fraction decreases to about 75%. Global VLBI data at 5 GHz show a compact component, a few milliarcsec in size (Stanghellini et al. 1997). The radio spectrum has a convex shape with possible variability around the peak occurring between 5 and 10 GHz.
0922+005 [Q, z = 1.72]
The radio spectrum of this quasar turned out to be flat between 5 and 0.4 GHz and therefore the turnover cannot be clearly determined. The radio emission is resolved at S band in PA . At X band again the source is just resolved in PA . This source was not detected at 2.3 MHz in a single baseline experiment by Morabito et al. (1986). Our VLBI images account for 55% and 65% of the total flux density at S and X bands respectively.
0941-080 [G, z = 0.228]
This galaxy (whose redshift has been provided by de Vries and O'Dea, private communication) is well resolved by the present data at both frequencies. The VLBI morphology is that of a compact double (see the images at both S and X-band shown in Fig. 2 (click here)), typical of many CSS-GPS radio galaxies. In our images and , clear indication of additional radio emission completely resolved out by the present observations, in particular at X band. If we consider the flux densities measured for the two components on our images, the spectral index is 1.0 and 1.4 for the southern and northern components respectively. A proper measurement would require matched uv-coverages; we might get spectral indices steeper than the true ones due to the poorer sampling of the short uv-spacings at X band. The radio spectrum peaks at about 0.4 GHz, and the high frequency spectral index is between 5.0 and 10.7 GHz (derived from the literature).
0941+261 [Q, z = 2.906]
This quasar has not been detected by the present observations at either frequency. The VLA image at 5 GHz by Barthel et al. (1988) shows a misaligned triple source. They identify the quasar with the brightest steep-spectrum component and therefore suggest the classification as curved-jet source. The integrated radio spectrum has a nearly constant slope between 0.4 and 5 GHz, with a spectral index of 0.6, although more measurements are required for a proper definition of the spectrum itself. The optical spectrum shows the presence of a number of absorption lines spread at various redshifts (see Hewitt & Burbidge 1989 for references).
1143-245 [Q, z = 1.950]
The radio spectrum is rather flat, although there are clear indications of a steepening at mm and sub-mm wavelengths (Steppe et al. 1992, 1995), and a sharp turnover around 1 GHz. The VLBI emission is dominated by a pointlike component, possibly barely resolved at S band, where . At X band the whole flux density of the source is accounted for in our data.
1237-101 [Q, z = 0.753]
This optically variable quasar has a flat low frequency radio spectrum without a well defined turnover, while there is some progressive steepening at cm and mm wavelengths. Some variability can also be inferred from the literature data spanning more than ten years in time. The mas radio emission is dominated by a compact region, barely resolved in PA . In our images and .
This object has also been detected in the X-rays (Wilkes et al. 1994).
1317-005 [Q, z = 0.892] (4C - 00.50)
The radio spectrum of this quasar is rather steep and does not show any turnover down to 80 MHz. The mas structure is dominated by a pointlike component at both frequencies, accounting for a limited fraction of the total flux density (15% at S and 25% at X band); such a straight radio spectrum would imply structures on scales larger than those found by the present observations.
1502+036 [Q, z = 0.411]
At cm wavelengths, the radio emission of this object turned out to be variable. At 5 GHz the flux densities reported in the literature differ by a factor of 2 at least. In fact Perley (1982) reports 0.47 Jy while in the Green Bank catalogue its flux density is given as 0.99 Jy and finally the updated flux density in the VLA calibrator manual is 0.90 Jy. Moreover, the flux density at 2.7 GHz in the new Parkes Catalogue is only of the flux density accounted for in our image. Our VLBI images are dominated by a very compact component, marginally resolved at S band, with an inverted spectral index.
1518+047 [G, z = 1.296] (4C + 04.51)
The radio spectrum of this galaxy peaks at about 0.8 GHz and then sharply declines at lower frequencies. The optically thin spectral index is rather steep, about 1.6. In Fig. 1 (click here) we did not plot the measurement at 80 MHz reported in the new Parkes Catalogue, because it is very high (16 Jy, to be compared with the 2 Jy measured at 178 MHz), possibly affected by confusion, since there is at least another strong source at about 1 arcmin from 1518+047 (see the VLA image by Rusk 1988). Some low frequency variability has been detected at 327 MHz by Ghosh et al. (1994).
The mas structure (see the images at both X and S-band in Fig. 2 (click here)) is dominated by two components separated by about 140 mas in PA , and they are likely to be the lobes of a medium-size compact symmetric object (MSO, Readhead et al. 1996). The northern region has a steeper spectral index () with respect to the southern (), although we must remark that at X-band we are likely to have detected the hot-spot region, and a proper comparison would require matched uv-coverages. In our images and . Our result is in basic agreement with Mutel et al. (1985), who presented images at 5 and 1.6 GHz, and resolve both components.
1607+268 [G, z = 0.473]
This galaxy, also known as CTD93, has a double VLBI morphology as already presented by Mutel et al. (1985) and various authors afterwards. It belongs to various samples of GPS and CSS sources (Fanti et al. 1990, 1995; O'Dea et al. 1991) and it was observed to test the reliability of the results obtained by the present observations. At both bands (see Fig. 2 (click here)) the source is dominated by two components with a flux density ratio of about 1 : 1.2, separated by about 48 mas (considering their centroids). The whole flux density is accounted for in the X band image, while . The radio spectrum peaks around 1 GHz, and the spectral index of the optically thin region is about 1.4. Moderate flux density variability has been revealed at 327 MHz by Ghosh et al. (1994).
1629+120 [Q, z = 1.795] (4C + 12.59)
The radio spectrum in Fig. 1 (click here) is steep (); however there are no flux density measurements at frequencies higher than 5 GHz, and therefore it is rather difficult to do a proper classification of its shape.
The arcsecond radio emission is that of a triple where the core is identified with the westernmost component of the VLA image by Barthel et al. (1988). Our VLBI image at S band shows two regions separated by arcsec; at X band only one of them has been detected. Since the westernmost component appears to be unresolved at S band, we tentatively say that component C1 (see Fig. 2 (click here)) is the flat spectrum core ( between our two images). The fraction of the total flux density accounted for in our images is generally small ( and we estimate ).
1848+283 [Q, z = 2.56]
The radio spectrum of this quasar is peaked around 10 GHz, and is characterized by a rather sharp falloff below 5 GHz. The radio polarization is also very low if not absent at cm wavelengths, as found by Rusk (1988). This evidence, together with the VLBI morphology, makes this object a positive GPS candidate. Possible variability can be inferred if we consider the 2 cm flux density reported by Rusk (1988) which is about 50% higher than the measurements found on the web page of the VLA calibrators manual and the flux density reported by Owen et al. (1980). Spangler et al. (1983) reported an increase of the high frequencies flux densities on a timescale of about 4 years; the radio spectrum was in both cases of GPS type.
Our images show an unresolved component at both frequencies, and while in the S band image only about 65% of the total flux is accounted for, in the X band the total flux density in the image exceeds (by about 20%) the average value derived from the literature, which is further indication of high frequency variability.
2044-027 [Q, z = 0.942] (3C 422, 4C - 02.80)
The radio spectrum of this quasar is typical of a CSS source, with a spectral peak of about 10 Jy occurring at about 200 MHz. The optical spectrum shows a number of absorption features occurring at a redshift very close to that of the quasar (Aldcroft et al. 1994). The radio emission is resolved with the VLA (see Price et al. 1993) at 6 cm, but it is still dominated by a compact component with 0.5 Jy.
Our S band image (Fig. 2 (click here)) shows well resolved emission with a lobe-like morphology, which accounts for about of the total flux density. The fits to the data and and to the image with a single component were rather poor and therefore we decided to fit the two components C1 and C2 listed in Table 4 (click here) and shown in Fig. 2. In the X band the source is barely detected, and the weak component (probably the hot-spot of the forementioned lobe) contains only about of the flux density of the source at this frequency.
2126-158 [Q, z = 3.268]
This high redshift radio quasar shows a large number of absorption features in the optical spectrum (see Junkkarinen et al. 1991), which has also been found to be very weakly polarized (Wills et al. 1992). The X-ray emission from this quasar is likely to be non variable (Elvis et al. 1994; Bechtold et al. 1994).
The radio spectrum is peaked around 4 GHz, although we cannot exclude some variability which can move the intrinsic spectral peak. At arcsecond resolution, Neff & Hutchings (1990) find a secondary component with about 5% of the total flux density at 20 cm, located at about 1.5 arcsec from the core region. They interpret this emission as a bright knot in a jet. Our VLBI images show a pointlike feature at both frequencies, with and . The radio source is resolved at mas resolution by Stanghellini et al. (1997) with a global VLBI observation at 5 GHz.
2128+048 [G z = 0.99]
The integrated spectrum of this weak galaxy is that typical of a GPS radio source, with a spectral peak occurring at about 600 MHz. The VLBI emission is well resolved into a triple (X band image), with a possible tail detected in the S band only (see Fig. 2 (click here)). The image at X band accounts for the whole flux density of the source, while the . If we assume that the source core is the central component C2 of the X band image, this object can be considered a CSO candidate. A global VLBI image at 5 GHz by Stanghellini et al. (1997) tentatively confirms this assumption.
2137+209 [Q, z = 1.576]
This quasar is resolved at both frequencies. In particular at S band (see Fig. 2) the VLBI morphology is rather complex and could be interpreted either in terms of a double or as a lobe with hot-spot. At both frequencies, our images account for about 50% of the total flux density. It is not easy to describe the integrated radio spectrum due to the scarcity of measurements; there might be some degree of variability at high frequencies as derived from the comparison between the flux density measurements at 6 cm from the Green Bank Catalogue (Becker et al. 1991) and the new Parkes Catalogue.
2210+016 [G, z not available] (4C + 01.69)
The optical identification for this source is still controversal. Stickel & Kühr (1996) find a quasar with m= 21.7 at R-band, while de Vries et al. (1995) find a galaxy with m=22.0 at i-band. We prefer the latter possibility, according to the VLBI morphology presented here, which is most commonly found among galaxies. The radio spectrum is typical of a CSS source, with a flattening of the slope at a few hundreds MHz. Ghosh & Rao (1992) find significant flux density variability at 327 MHz, while there are no indications of variability at high frequencies.
The VLBI structure revealed in our images is rather complex (Fig. 2 (click here)). At S band the radio emission is dominated by three components (accounting for about 65% of the total flux density) which are further resolved into a number of components in the X band image (where ). A comparison with a global VLBI image at 6 cm by Stanghellini et al. (1997) provides a confirmation of the forementioned structure, although it is not clear which component can be considered the source core. The most likely candidate is component C1, which is the most compact one.
2223+210 [Q, z = 1.959]
Aldcroft et al. (1994) detected a number of absorption features at and in the optical spectrum. There is also a positive detection in the X-rays (Wilkes et al. 1994).
The integrated radio spectrum reveals possible variability of this quasar (as can be derived from a comparison between the 2.7 GHz flux densities reported in Kühr et al. 1981 and in the new Parkes catalogue). It is difficult to determine the optically thin spectral index, since we would need simultaneous measurements at the various frequencies, however it might be consistent with a CSS type, with a possible turnover at about 400 MHz.
The VLA image at 5 GHz by Barthel et al. (1988) shows a secondary weak component at about in PA 30, responsible for a small fraction of the total flux density, and a deeper image by Lonsdale et al. (1993) shows also a third component at about on the opposite side, although it might also be an unrelated object.
The mas structure is dominated by a single component, which is barely resolved at S band. The total flux density in the X band exceeds the measurement reported in the Parkes catalogue by about 30%. On the other hand, the S band image accounts for about half of the total flux density.
2351-006 [Q, z = 0.46]
This object has been identified with a quasar at relatively low redshift (Hewitt et al. 1995). The EVN images at both frequencies are dominated by a pointlike component accounting for about two thirds of the total flux density. The information available in the literature is rather sparse; consequently, the integrated radio spectrum has very few points and does not allow a proper classification.
As mentioned in Sect. 2, in May 1994 we observed 4 objects, spending about 2 hrs on each source. This allowed a better filled uv-plane and therefore a higher image fidelity.
1402-012 [Q, z = 2.518]
This high redshift quasar has been reported to be optically variable (Pica et al. 1988), and to have a number of absorption features in the optical spectrum (Junkkarinen et al. 1991). Moreover is has also been detected by the Einstein satellite in the X-rays (Wilkes et al. 1994).
The radio flux densities collected from the literature do not allow a complete determination of the radio spectrum. The mas radio emission is dominated by a single component at both frequencies, barely resolved at X band, where the whole flux density is accounted for. On the other hand at S band only about 75% of the total flux is present in the image (not shown).
1602+576 [Q, z = 2.85] (4C + 57.27)
The S band image shows two asymmetric components separated by about 120 mas. The flux density ratio is about 25 : 1. The X band image shows a single component likely to be identified with the brighter one at X band. Both images account for the whole flux density of the radio source at each frequency respectively. The integrated spectrum is poorly defined due to the lack of measurements in the literature.
1629+680 [Q, z = 2.475] (4C + 68.18)
This quasar shows an absorption feature in the optical spectrum (Junkkarinen et al. 1991). The radio spectrum is straight and rather steep. The VLBI images show a well resolved jet-like structure. The size of the radio emission is 50 mas at 2.3 GHz, with indications for a possible bend. The structure imaged at 8.3 GHz is much shorter due to the lower surface brightness and less sensitivity to the extended structure. This source has been found to be unresolved (angular size smaller than 0.25 arcsec) with MERLIN at 408 MHz and with the VLA at 5 GHz by Reid et al. (1995)
1801+010 [Q, z = 1.522]
This source was taken from Barthel et al. (1988), and has a rather flat spectrum which steepens at about 5 GHz. Some variability can also be inferred from literature data. Our VLBI images are dominated by a pointlike component. The spectral index between S and X bands is 0.34. Simard-Normandin et al. (1981) found 13% fractional polarization at L band.