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VLBI observations of the quasar 3C380
Izabela
Owsianik
e-mail:
iza@astro.uni.torun.pl
Torun Radio Observatory, Nicolaus Copernicus University, POLAND
Abstract:
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.
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 
).
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):
- 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 (
).
- the PA of the apparent velocity vector changes from
to 
degree.
- 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)).
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.
- Fanaroff B.L., Riley J.M., 1974,
MNRAS, 167, 31.
- Fanti R., Fanti C., Schilizzi R.T., Nan Rendong, Parma P.,
van Breugel W.J.M., Venturi T., 1990,
A&A, 231, 333.
- Kapahi V.K., 1981,
A&AS, 43, 381.
- Kus A.J., Booth R.S., Marecki A., Maszkowski R., Porcas R.W.,
Pearson T.J., Readhead A.C.S., Wilkinson P.N., 1993,
in Sub-arcsecond Radio Astronomy, eds Davis R.J. & Booth R.S.,
p.222, (Cambridge University Press).
- Peacock J.A., Wall J.V., 1982,
MNRAS, 198, 843
- Readhead A.C.S., 1993,
in Sub-arcsecond Radio Astronomy, eds Davis R.J. & Booth R.S.,
p.173, (Cambridge University Press).
- Sanghera H.S., 1992,
PhD thesis, (University of Manchester).
- Wilkinson P.N, Booth R.S., Cornwell T.J., Clark R.R., 1984,
Nature, 308, 619.
- Wilkinson P.N., Chidi E. Akujor, Cornwell T.J., Saikai D.J., 1991,
MNRAS, 248, 86 .
YERAC 94 Account
Wed Feb 22 21:00:54 GMT 1995
