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VLBI observations of the quasar 3C380

Izabela Owsianik

Torun Radio Observatory, Nicolaus Copernicus University, POLAND


There are result from two global VLBI observations at 3.6 cm spaced about 2 years. These observations were made in epochs 1989.9 and 1992.25 and they are unique at 8.4 GHz frequency. The maps were made with 0.5 mas resolution and show dramatic changes in the structure over the period. The analysis of the apparent motion, indication of the jet flow trajectory and the other implications for the CSS quasars are discussed.


1. Introduction

The quasar 3C380 (; thus 1 arcsec corresponds to 4 kpc for , ) is one of the most powerful radio sources (its radio luminosity at 2.7 GHz is ) (Wilkinson et al. (1984)). 3C380 is a member of the class of Compact Steep Spectrum (CSS) sources. CSS objects are those sources, which are unresolved or barely resolved by conventional interferometry (angular size ) and have a steep high frequency spectrum (spectral index , with ) (Kapahi (1981),Peacock & Wall (1982)). Their occurrence is quite high in catalogues of radio sources (from 15% to 30%, depending on selection frequency). For many years they were not studied intensively, because of their uninteresting structure at arcsec resolution. At resolutions of hundreds to tens of arcsec, CSSs show the same variety of structure as the more extended radio sources, i.e. double, triple and core-jet (Fanti et al. (1990)). In addition, very complex structures reminiscent of sharply bent jets are also observed.

The majority of the powerful steep-spectrum sources have FR II structures (Fanaroff & Riley (1974)), i.e. two dominant edge-brightened lobes straddling a relatively weak core. However, 3C380 does not fit to this pattern. Instead it has a relatively strong core, which dominates the spectrum above GHz, surrounded by a tangle of emission whose underlying structure is far from clear (Wilkinson et al. (1984)). 3C380 is classified as a CSS source despite its extended structure (angular size arcsec), since most of its radio emission at 5 GHz apparently comes from a region only a few arcsec in extent, which is not typical of FR II sources. Quasar 3C380 shows complex or very asymmetric structure on the kilo-parsec scale, whereas on the parsec scale it belongs to the one-sided jet morphology.

Figure 1: Global VLBI maps of 3C380 at 8.4 GHz made for epochs 1989.9 and 1992.25. Contour levels are 0.5, 1, 2, 4, 8, 16, 32, and 64% of the peak brightness (about ).

2. Observational results

VLBI observations of the quasar 3C380 were made with 8-telescopes on the 19th November 1989 and with 13-telescopes on the 1st April 1992. Data were correlated in OVRO Caltech by Dr Faith Austin, then reduced with Astronomical Image Processing System (AIPS). The maps were made with 0.5 mas resolution and show dramatic changes in the structure over the period (2.36 yrs).

Figure 2: The apparent velocity for all components of 3C380 derived from maps.

The detailed comparison of both maps leads to the following conclusions (see Figure 2):

  1. Apparent velocities have been measured on the scale mas, it ranges from to . Component C3 has moved with the same velocity as the component A ().
  2. the PA of the apparent velocity vector changes from to degree.
  3. The 8 mas long jet rotates anticlockwise at about (measured at the component C0).

The quasar 3C380 is a very active, very strong and it shows a large range of variability. The observed motion is most probably a combination of the ballistic (near the core) and helical type in the distant regions (Kus et al. (1993)).

3. Discussion

Initial disturbance in the jet direction can account for some of the observed features. Without the existence of inhomogeneities in the dense and/or turbulent medium it is very difficult to propose a realistic physical phenomenon which would explain the observed facts (Readhead (1993)). 3C380 exhibits several features expected of FR II sources seen approximately end-on, like superluminal motion in the core, the similarity of the NW knots to double hot spots in FR II lobes and so on. However, not all of 3C380 futures can be so easily accounted for by projection to the line of sight, in particular the core is relatively weak and it is not strongly variable at radio and optical wavelengths (Wilkinson et al. (1991)).

Radio statistics suggest that CSS objects are rather intrinsically small sources of subgalactic size. High resolution observations reveal a striking similar radio structure to that observed in the extended sources, with jets and lobes, and invariably displaying a flat spectrum radio core at high frequencies. The age of CSS's () implies that they are represent a young evolutionary stage of the large radio sources, in which the radio jets are still propagating through the interstellar medium. Projection and beaming effects are a possible explanation for the strong cores and jets in CSS quasars, which also are generally more asymmetric. Asymmetries are probably due to differences in the density of the ambient medium, and the interaction effects between the radio emitting plasma and the surrounding medium are likely to dominate the appearance of the radio structure. Evidence that CSSs inhabit a denser and possibly clumpier medium is shown by their more misaligned and asymmetric radio structures than are observed in the extended sources (Sanghera (1992)).

There is strong need to continue the VLBI observations of quasar 3C380 at high frequency with better sensitivity. More frequent, regular study of structure changes should provide us with the new information on detailed evolution of the jets and physics of such phenomena. Our proposal to continue VLBI monitoring of quasar 3C380 was accepted, and we hope that it will help us to built up a theory of interaction with the environment.


I thank all people for proposal of VLBI observations of this quasar. They are R.S. Booth Onsala Space Observatory, T.J. Pearson and A.C.S. Readhead OVRO Caltech, P.N. Wilkinson NRAL Jodrell Bank and specially my supervisor A.J. Kus TRAO Torun for all help.


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Wed Feb 22 21:00:54 GMT 1995