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4. Results and comments on individual sources

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 5tex2html_wrap_inline1700 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.

  figure263  figure270  figure277  figure283
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.

4.1. Session 1

As mentioned in Sect. 2, Noto could not record data at S-band, and the visibilities to Onsala were of no use at X-band.

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 (tex2html_wrap_inline1740). 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 Lytex2html_wrap_inline1742 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 tex2html_wrap_inline1756 (tex2html_wrap_inline1758). 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 (tex2html_wrap_inline1770), 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 tex2html_wrap_inline1774 90tex2html_wrap_inline1776 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 tex2html_wrap_inline1770), at X band (Fig. 2) the radio emission shows two very asymmetric regions separated by tex2html_wrap_inline1774 10 mas in PA tex2html_wrap_inline1794 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 (tex2html_wrap_inline1796 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 tex2html_wrap_inline1806 and can be either a core-jet or an asymmetric triple (tex2html_wrap_inline1808). The S band image instead shows unresolved emission with tex2html_wrap_inline1812 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.

  figure321  figure330  figure336  figure342
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 tex2html_wrap_inline1840. At X band again the source is just resolved in PA tex2html_wrap_inline1844. This source was not detected at 2.3 MHz in a single baseline experiment by Morabito et al. (1986). Our VLBI images account for tex2html_wrap_inline1774 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 tex2html_wrap_inline1862 and tex2html_wrap_inline1864, 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 tex2html_wrap_inline1874 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 tex2html_wrap_inline1890. 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 tex2html_wrap_inline1900. In our images tex2html_wrap_inline1902 and tex2html_wrap_inline1904.

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 (tex2html_wrap_inline177415% at S and tex2html_wrap_inline177425% 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 tex2html_wrap_inline1928 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 tex2html_wrap_inline1942, 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 (tex2html_wrap_inline1944) with respect to the southern (tex2html_wrap_inline1946), 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 tex2html_wrap_inline1952 and tex2html_wrap_inline1954. 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 tex2html_wrap_inline1964. 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 (tex2html_wrap_inline1972); 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 tex2html_wrap_inline1976 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 (tex2html_wrap_inline1982 between our two images). The fraction of the total flux density accounted for in our images is generally small (tex2html_wrap_inline1984 and we estimate tex2html_wrap_inline1986).

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.

 

Name Comp. r tex2html_wrap_inline1998 S Maj.ax. Min.ax. PA Peak1tex2html_wrap_inline2002 lev.
mas degrees Jy mas mas degrees Jy mJy
(1) (2) (3)(4)(5) (6) (7) (8) (9) (10)
0201+113 tex2html_wrap_inline2006 <3 <3
0237-027 tex2html_wrap_inline2014 <2.5 <2.5
0237-233 C1 tex2html_wrap_inline2022 13.1 1.5 tex2html_wrap_inline2024 1.18 15
C2 tex2html_wrap_inline2026tex2html_wrap_inline2028tex2html_wrap_inline2030 11.0 4.4 tex2html_wrap_inline2032
0500+019 C1 tex2html_wrap_inline2036tex2html_wrap_inline2038<2.5tex2html_wrap_inline2042 0.90 4
C2 tex2html_wrap_inline2044tex2html_wrap_inline2046 tex2html_wrap_inline2048tex2html_wrap_inline2050tex2html_wrap_inline2052tex2html_wrap_inline2054
0511-220 C1 tex2html_wrap_inline2058tex2html_wrap_inline2060<2.5tex2html_wrap_inline2064 0.73 7
C2 tex2html_wrap_inline2066tex2html_wrap_inline2068 tex2html_wrap_inline2070tex2html_wrap_inline2072tex2html_wrap_inline2074tex2html_wrap_inline2076
0743-006 tex2html_wrap_inline2080<2.5<2.5
0922+005 tex2html_wrap_inline2088tex2html_wrap_inline2050<2.5tex2html_wrap_inline2094
0941-080 C1 tex2html_wrap_inline2098tex2html_wrap_inline2100tex2html_wrap_inline2074tex2html_wrap_inline2104 0.076 5
C2 tex2html_wrap_inline2106tex2html_wrap_inline2108 tex2html_wrap_inline2110tex2html_wrap_inline2112tex2html_wrap_inline2114tex2html_wrap_inline2116
1143-245 tex2html_wrap_inline2120<3<3
1237-101 tex2html_wrap_inline2128<2.5<2.5
1317-005 tex2html_wrap_inline2048<2.5<2.5
1402-012 tex2html_wrap_inline2144tex2html_wrap_inline2146<1.5tex2html_wrap_inline2150
1502+036 tex2html_wrap_inline2154<4<4
1518+047 C1 tex2html_wrap_inline2162tex2html_wrap_inline2164tex2html_wrap_inline2166tex2html_wrap_inline2168 0.16 5
C2 tex2html_wrap_inline2170tex2html_wrap_inline2172 tex2html_wrap_inline2174tex2html_wrap_inline2176tex2html_wrap_inline2178tex2html_wrap_inline2180
1602+576 C1 tex2html_wrap_inline2184<1.5<1.5 0.16 3
C2 tex2html_wrap_inline2190tex2html_wrap_inline2192tex2html_wrap_inline2110tex2html_wrap_inline2196 tex2html_wrap_inline2198tex2html_wrap_inline2200
1607+268C1 tex2html_wrap_inline2204tex2html_wrap_inline2206tex2html_wrap_inline2208 tex2html_wrap_inline2210 0.40 5
C2tex2html_wrap_inline2212tex2html_wrap_inline2214tex2html_wrap_inline2216tex2html_wrap_inline2176tex2html_wrap_inline2220tex2html_wrap_inline2222
1629+120 tex2html_wrap_inline2226tex2html_wrap_inline2228<1.5tex2html_wrap_inline2232
1629+680 tex2html_wrap_inline2236tex2html_wrap_inline2238tex2html_wrap_inline2240tex2html_wrap_inline2242
1801+010 C1 tex2html_wrap_inline2246tex2html_wrap_inline2248<1.5tex2html_wrap_inline2252 0.90 5
C2tex2html_wrap_inline2178tex2html_wrap_inline2256tex2html_wrap_inline2258tex2html_wrap_inline2074tex2html_wrap_inline2262tex2html_wrap_inline2264
1848+283 tex2html_wrap_inline2268<2.5<2.5
2044-027 tex2html_wrap_inline2276<3<3
2126-158 tex2html_wrap_inline2284<4<4
2128+048 C1 tex2html_wrap_inline2246tex2html_wrap_inline2038<2tex2html_wrap_inline2298 0.79 4
C2tex2html_wrap_inline2300tex2html_wrap_inline2302tex2html_wrap_inline2304tex2html_wrap_inline2306<2tex2html_wrap_inline2310
C3tex2html_wrap_inline2312tex2html_wrap_inline2314tex2html_wrap_inline2316tex2html_wrap_inline2318<1.5tex2html_wrap_inline2322
2137+209 tex2html_wrap_inline2326tex2html_wrap_inline2328<1.5tex2html_wrap_inline2076
2210+016 C1 tex2html_wrap_inline2336tex2html_wrap_inline2338<1.5tex2html_wrap_inline2116 0.068 4
C2tex2html_wrap_inline2344tex2html_wrap_inline2346tex2html_wrap_inline2348tex2html_wrap_inline2350tex2html_wrap_inline2352tex2html_wrap_inline2354
C3tex2html_wrap_inline2356tex2html_wrap_inline2358tex2html_wrap_inline2360tex2html_wrap_inline2362tex2html_wrap_inline2364tex2html_wrap_inline2366
2223+210 tex2html_wrap_inline2370<3<3
2351-006 C1 tex2html_wrap_inline2378<3<3 0.25 3
C2tex2html_wrap_inline2384tex2html_wrap_inline2386tex2html_wrap_inline2276tex2html_wrap_inline2364tex2html_wrap_inline2038tex2html_wrap_inline2394
Table 3: Source models at X band (8.3 GHz). Column 1 gives the source name, Col. 2 shows the component name derived from the images of Fig. 2 (click here); Cols. 3 and 4 show the radial distance and position angle of the various component derived with respect to component C1. Column 5 gives the flux density; Cols. 6, 7 and 8 the major axis, the minor axis and the position angle of each component. Finally, Cols. 9 and 10 give the peak intensity and the first level contour in case an image of the source is shown in Fig. 2

 

 

Name Comp. r tex2html_wrap_inline1998 S Maj.ax. Min.ax. PA Peak1tex2html_wrap_inline2002 lev.
mas degrees Jy mas mas degrees Jy mJy
(1) (2) (3)(4)(5) (6) (7) (8) (9) (10)
0201+113 tex2html_wrap_inline2408 <7 <7
0237-027 tex2html_wrap_inline2416 tex2html_wrap_inline2418<6tex2html_wrap_inline2422
0237-233 tex2html_wrap_inline2426<5 <5
0500+019 tex2html_wrap_inline2434tex2html_wrap_inline2436<8tex2html_wrap_inline2440
0511-220 tex2html_wrap_inline2444<6<6
0743-006 tex2html_wrap_inline2452<8<8
0922+005 tex2html_wrap_inline2460tex2html_wrap_inline2220<6tex2html_wrap_inline2466
0941-080 C1 tex2html_wrap_inline2470tex2html_wrap_inline2472tex2html_wrap_inline2176tex2html_wrap_inline2476 0.38 6
C2 tex2html_wrap_inline2478tex2html_wrap_inline2480 tex2html_wrap_inline2482tex2html_wrap_inline2484tex2html_wrap_inline2176tex2html_wrap_inline2488
1143-245 tex2html_wrap_inline2492tex2html_wrap_inline2176<13tex2html_wrap_inline2498
1237-101 tex2html_wrap_inline2502tex2html_wrap_inline2176<8tex2html_wrap_inline2508
1317-005 tex2html_wrap_inline2512<7<7
1402-012 tex2html_wrap_inline2204<6<6
1502+036 tex2html_wrap_inline2528tex2html_wrap_inline2530<8tex2html_wrap_inline2534
1518+047C1 tex2html_wrap_inline2538tex2html_wrap_inline2540tex2html_wrap_inline2542tex2html_wrap_inline2544 0.72 6
C2tex2html_wrap_inline2546tex2html_wrap_inline2548 tex2html_wrap_inline2550tex2html_wrap_inline2552tex2html_wrap_inline2554tex2html_wrap_inline2556
1602+576C1+C2 tex2html_wrap_inline2560<5<5 0.51 6
C3tex2html_wrap_inline2566tex2html_wrap_inline2568tex2html_wrap_inline2570tex2html_wrap_inline2572tex2html_wrap_inline2074 92tex2html_wrap_inline2576
C4tex2html_wrap_inline2578tex2html_wrap_inline2580tex2html_wrap_inline2582tex2html_wrap_inline2584tex2html_wrap_inline2178tex2html_wrap_inline2588
1607+268C1 tex2html_wrap_inline2592tex2html_wrap_inline2594<3tex2html_wrap_inline2054 1.18 6
C2tex2html_wrap_inline2600tex2html_wrap_inline2214tex2html_wrap_inline2492tex2html_wrap_inline2176tex2html_wrap_inline2608tex2html_wrap_inline2610
1629+120 C1 tex2html_wrap_inline2226<10<10 0.36 5
C2tex2html_wrap_inline2620tex2html_wrap_inline2622tex2html_wrap_inline2624tex2html_wrap_inline2626tex2html_wrap_inline2628tex2html_wrap_inline2630
1629+680 C1 tex2html_wrap_inline2634tex2html_wrap_inline2636<5tex2html_wrap_inline2640 0.41 7
C2tex2html_wrap_inline2642tex2html_wrap_inline2644tex2html_wrap_inline2646tex2html_wrap_inline2648tex2html_wrap_inline2650tex2html_wrap_inline2652
1801+010 tex2html_wrap_inline2656tex2html_wrap_inline2658tex2html_wrap_inline2572tex2html_wrap_inline2662
1848+283 tex2html_wrap_inline2666<7<7
2044-027C1 tex2html_wrap_inline2674tex2html_wrap_inline2676tex2html_wrap_inline2678tex2html_wrap_inline2680 0.14 6
C2 tex2html_wrap_inline2682tex2html_wrap_inline2684tex2html_wrap_inline2686 tex2html_wrap_inline2688tex2html_wrap_inline2690tex2html_wrap_inline2692
2126-158 tex2html_wrap_inline2696<10<10
2128+048 C1+C2 tex2html_wrap_inline2704tex2html_wrap_inline2706<7tex2html_wrap_inline2710 1.55 6
C3tex2html_wrap_inline2712tex2html_wrap_inline2714tex2html_wrap_inline2716tex2html_wrap_inline2472tex2html_wrap_inline2720tex2html_wrap_inline2722
C4tex2html_wrap_inline2724tex2html_wrap_inline2726tex2html_wrap_inline2728tex2html_wrap_inline2730tex2html_wrap_inline2594tex2html_wrap_inline2734
2137+209 C1 tex2html_wrap_inline2738tex2html_wrap_inline2740<7tex2html_wrap_inline2744 0.42 6
C2tex2html_wrap_inline2746tex2html_wrap_inline2748tex2html_wrap_inline2360tex2html_wrap_inline2752tex2html_wrap_inline2754tex2html_wrap_inline2116
2210+016 C1+C2 tex2html_wrap_inline2760tex2html_wrap_inline2762tex2html_wrap_inline2764tex2html_wrap_inline2766 0.36 6
C3 tex2html_wrap_inline2768tex2html_wrap_inline2358tex2html_wrap_inline2772tex2html_wrap_inline2774tex2html_wrap_inline2220tex2html_wrap_inline2778
C4tex2html_wrap_inline2780tex2html_wrap_inline2782tex2html_wrap_inline2784tex2html_wrap_inline2786tex2html_wrap_inline2786
2223+210 tex2html_wrap_inline2792tex2html_wrap_inline2026tex2html_wrap_inline2178tex2html_wrap_inline2798
2351-006 tex2html_wrap_inline2802tex2html_wrap_inline2074<7tex2html_wrap_inline2808
Table 4: Source models at S band (2.3 GHz). Column 1 gives the source name, Col. 2 shows the component name derived from the images of Fig. 2 (click here); Cols. 3 and 4 show the radial distance and position angle of the various component derived with respect to component C1. Column 5 gives the flux density; Cols. 6, 7 and 8 the major axis, the minor axis and the position angle of each component. Finally, Cols. 9 and 10 give the peak intensity and the first level contour in case an image of the source is shown in Fig. 2

 

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 tex2html_wrap_inline2824 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 tex2html_wrap_inline2828 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 tex2html_wrap_inline2836 and tex2html_wrap_inline1904. 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 tex2html_wrap_inline1954. 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 tex2html_wrap_inline2884). 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 tex2html_wrap_inline2892 and tex2html_wrap_inline2894 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 tex2html_wrap_inline2896 in PA 30tex2html_wrap_inline1776, 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 tex2html_wrap_inline2900 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.

4.2. Session 2

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 tex2html_wrap_inline1774 50 mas at 2.3 GHz, with indications for a possible tex2html_wrap_inline2960 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.

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