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Astron. Astrophys. 344, 51-60 (1999)

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4. Discussion

Frey et al. (1997) studied the parsec scale structural properties of radio loud QSO's using a sample of 151 quasars in the redshift range of [FORMULA] observed with sufficiently high resolution at 5 GHz. They determined the flux density ratios of the brightest "jet" and "core" components ([FORMULA]/[FORMULA]) of the sources. The typical angular resolution of those VLBI observations was [FORMULA]1 mas. Because the linear resolution is better for the lowest redshift sources, they introduced a linear size limit to distinguish between jet and core components in order to compare the same linear sizes at different redshifts. One milliarcsecond sets the linear resolution to 7 pc for [FORMULA] sources up to the highest redshifts represented in the sample ([FORMULA]=80 km s- 1 Mpc-1 and [FORMULA]=0.1 were used to calculate linear sizes; the angular size of a fixed linear size is practically constant at [FORMULA] for plausible cosmological models). The value of 7 pc was not used in any quantitative way in their analysis, just as a threshold between cores and jets. Only components outside the core region were considered as jet components. They found a weak overall trend of a decreasing jet to core flux density ratio with increasing redshift which may be explained by the combined effect of the shifts of the emitting frequencies at different redshifts compared to the 5 GHz observing frequency and the different characteristic spectral indices in cores and jets.

We followed Frey et al. (1997) and calculated the jet to core flux density ratios ([FORMULA]/[FORMULA]) for the [FORMULA] sources presented in this paper (last column of Table 4). We had to exclude two sources from the analysis, 0004+139 and 1500+045. In the former case, the angular resolution in the direction of the expected jet structure is considerably greater than 1 mas. The quasar 1500+045 may also have jet structure which is not observable in our data due to the unfortunate orientation of the 10.5 mas restoring beam. The [FORMULA]/[FORMULA] values for the other three sources observed in the 1992 global experiment (0830+101, 0906+041 and 0938+119) should also be interpreted with caution since the restoring beam is very elongated. However, we derived tentative [FORMULA]/[FORMULA] values because the direction of the jet structure indicated by our maps are nearly perpendicular to the major axis of the beam and the resolution in this direction is about 1 mas. In the case of 0938+119 and 1428+423, an upper limit of the jet flux density was calculated based on the beam sizes and the 3[FORMULA] RMS noises on our images.

We added our eight new sources to the sample of Frey et al. (1997). The median [FORMULA] values as function of redshift are shown in Fig. 3. The data for all 159 sources are evenly grouped into 13 bins. Error bars indicate the mean absolute deviation of data points from the median within each bin. Upper limits and measured values are treated similarly. However, the plotted error bars are indicative of the scatter of the data. The solid curve represents the best least squares fit based on the 13 median values. Under the assumption that intrinsic spectral properties at the sources could be described by simple power-law dependence, the average difference between jet and core spectral indices can be estimated as [FORMULA].

[FIGURE] Fig. 3. Median jet to core flux density ratios versus redshift for 159 quasars of Frey et al. (1997) and this paper. Values are grouped into 13 nearly equally populated bins (12-13 sources per bin). The solid curve represents the best fit to the 13 median values. Circles indicate the upper limits of [FORMULA] for the three [FORMULA] quasars imaged with VLBI at 5 GHz to date (see Sect. 4.)

The three circles at the high redshift end of the plot in Fig. 3 show the upper limits of the jet to core flux density ratios for the most distant ([FORMULA]) quasars imaged at 5 GHz with VLBI to date. The sources 1251-407 ([FORMULA], Shaver et al. 1996) and 1428+423 ([FORMULA], Hook & McMahon 1998) are represented by filled circles. The open circle corresponds to the quasar 1508+572 ([FORMULA], Hook et al. 1995) which also appeared to be unresolved, however, at a considerably lower angular resolution ([FORMULA] mas) than the other sources included in the sample (Frey et al. 1997).

We note that in both cases available to date, radio structures in quasars at [FORMULA] (1251-407 and 1428+423) appear to be unresolved with a nominal resolution of [FORMULA]1 mas. The third case, 1508+572, albeit with a lower resolution of 5 mas, does not show a jet-like structure either. Qualitatively, it is consistent with the overall trend that steeper spectrum jets are fainter relative to flat spectrum cores at higher redshift because the fixed 5 GHz observing frequency implies high rest-frame frequency (for [FORMULA] the emitted frequency [FORMULA] GHz). However, these sources appear to be much more compact than expected from the general trend shown in Fig. 3. Even in the neighboring high redshift bins ([FORMULA]), it is unlikely that we find 3 randomly selected sources practically unresolved. A possible explanation for the observed compactness is that the spectral indices of the jet components become steeper with frequency, which results in a relative fading of the components with respect to the core at the high emitting frequencies ([FORMULA]25 GHz) of the largest redshift sources. Future multi-frequency VLBI observations of more [FORMULA] radio loud quasars with the highest possible sensitivity and angular resolution should answer the question whether these objects are indeed intrinsically so compact or there is a strong observational selection effect responsible for their particularly compact appearance.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999