Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/yerac/hardcastle/hardcastle.ps Äàòà èçìåíåíèÿ: Wed Feb 22 21:17:30 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 04:51:13 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: jet

Jets in nearby FRII radio galaxies
By M a r t i n H a r d c a s t l e
e­mail: mjh22@mrao.cam.ac.uk
Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge
CB3 0HE, UNITED KINGDOM
Jets are now detected in up to 70% of a sample of 50 nearby FRII radio galaxies with z ! 0:3
taken from the 3CR catalogue, the majority of which have been imaged at 8 GHz with the VLA:
this represents a significant increase in detection rates over previous systematic jet searches. The
jets in the sample are almost exclusively one­sided, and show a remarkable range of types of
structure. Preliminary attempts to relate jet detection rate to parameters such as the total
source luminosity have proved unsuccessful, suggesting a strong environmental influence on jet
visibility. I discuss further work to be done on this sample of sources, and the relation of our
results to other extragalactic objects with jets.
1. Brief introduction: what is a jet?
High­quality radio images show narrow linear features linking central `cores' to exten­
ded `lobes' in many radio galaxies and quasars, and these have become known as jets.
Bridle & Perley (1984) were the first to state exactly what they meant by `jet' in their
well­known review paper. According to Bridle and Perley, a jet is
ffl at least four times as long as it is wide
ffl separable at high resolution from other extended structures
ffl aligned with the compact radio core where it is closest to it
and they extended this definition to trains of knots if they contained more than two
knots or if some knots were elongated along the jet axis. These jets were identified with
the `beams' carrying energetic particles from the central engine out into the lobes.
2. Jets in FRII radio galaxies: the problem
New images presented new problems, however. Kiloparsec­scale jets were not detected
in the vast majority of FRII radio galaxies; as recently as 1991 Muxlow and Garrington
found that only 10% of these objects had known jets, while FRII quasars of similar
power almost all have bright one­sided jets (FRI radio galaxies normally have two­sided
jets). This is puzzling if we believe that the jets trace the energy transport in the radio
source: it is also puzzling if we believe that quasars and radio galaxies are essentially the
same sort of object, as many of their other large­scale properties would suggest (Barthel
(1994)). The standard explanation is that the jets in quasars are relativistic and close to
the line of sight: this explains their extreme one­sidedness. In this simple unified scheme,
radio galaxies would be in the plane of the sky and their jets would be dimmer and less
one­sided than those of quasars. However, this model remains no more than speculation
without observations of a large number of dim jets in FRII radio galaxies.
3. Jets in FRII radio galaxies: the observations
Recently these have started to become available. Black et al. (1992) looked at a sample
of nearby (z ! 0:15) powerful (P 178 ? 1:5 \Theta 10 25 W Hz \Gamma1 sr \Gamma1 ) radio galaxies from the
3CR catalogue, imaging them with the full VLA at 8 GHz, and detected jets in up to
1

2 Martin Hardcastle: Jets in nearby FRII radio galaxies
Figure 1. 3C438 at 0.75 arcsec resolution. Contours at 400 ¯Jy beam \Gamma1 \Theta (1;
p
2; 2; 2
p
2; : : :).
70% of them. Typically the contrast between the surface brightness of the jet and the
surrounding lobe was only ¸ 5, so that high resolution and sensitivity were needed. Over
the limited luminosity range available to him, Black found no relationship between the
detectability and brightness of the jets and the luminosity of the parent sources.
To extend the work of Black et al., we have embarked on a project to map the galaxies
in the LRL sub­sample of 3CR (Laing, Riley & Longair 1983) with 0:15 ! z ! 0:30.
We have obtained 8 GHz VLA data on 17 of the 21 objects in this redshift range (the
remaining four are large and well­studied) at B, C and D configurations, and we expect to
obtain A configuration data where necessary. As we expected, the maps show a number
of jets, and --- like the objects in Black's sample --- the features which we describe as jets
are remarkably different in appearance, ranging from the twin jets in 3C438 (Figure 1)
and the bright one­sided jet in 3C401 to thin, barely perceptible features like the jet in
3C20 (Figure 2).
Our results so far confirm the findings of Black et al. that up to 70% of these sources
have identifiable jets, and provide a wealth of information on individual objects. But
what can they tell us about the physics of jets in general?
4. Jets in FRII radio galaxies: the physics?
It is immediately obvious from the maps that the jets have a wide range of luminosities,
even among sources with similar overall power (compare e.g. 3C401 and 3C436) which
highlights the question ``what makes these jets light up?''. Although this large sample of
FRII jets will allow us to try to relate the luminosity of the jet to the source luminosity
and to properties known to be indicators of the beam power, such as [OIII] narrow line
luminosity (Rawlings & Saunders (1991)), it's clear from preliminary work (Figure 3)
that there will be a great deal of scatter in such a relation: some additional variables

Martin Hardcastle: Jets in nearby FRII radio galaxies 3
Figure 2. 3C20 at 0.6 arcsec resolution. Contours as for Figure 1. The jet appears as a
curving ridge in the western lobe.
are affecting the fraction of the beam power channelled into jet synchrotron radiation.
The fact that jets can bend and change dramatically in surface brightness along their
length is suggestive of a strong interaction with the environment, and some authors (e.g.
Fraix­Burnet (1992)) have argued that this interaction is entirely responsible for the
variations in jet properties. We know that environmental asymmetries can dramatically
influence the large­scale properties of a radio source from the work of McCarthy, van
Breugel & Kapahi (1991), who showed that the extended emission­line regions in these
sources, where their properties are known, are brighter on the side with the shorter
lobe; this ties in with the well­known result that the brighter lobe in these sources has
a weak but definite tendency to be the shorter. The problem with applying this to jets
is that na¨ive models make predictions --- that the brighter or only jet should lie in the
shorter or longer lobe --- which are not borne out by observation. One result of Black's
which seems to be confirmed by our observations is that jets are more readily detected
in smaller sources (Figure 4), which seems to be evidence for the large­scale environment
influencing the jet visibility. We have applied for time on the Hubble Space telescope
to make high­sensitivity observations of the host galaxies of sources in our sample: this
may provide an important handle on the environment problem.
Another result of Black's which is confirmed by our extension of his sample is the one­
sided nature of these jets. With the exception of the remarkable twin­jet source 3C438
--- the most powerful such source known --- there are no definite counterjets in the new
sample and surprisingly few features that could be said to resemble them. The origin of
this one­sidedness is debatable. The three standard suggestions to explain it are
(a) The jets are intrinsically symmetrical in luminosity in their rest frames, but have
relativistic speeds; the jet is Doppler boosted towards us and the counterjet is boosted
away.
(b) The beams are intrinsically symmetrical, but the fraction of the beam channelled
into synchrotron emission is different on the two sides of the source.
(c) The beams are not intrinsically symmetrical: there is really much less going on on
the counterjet side of the source; but the beam power varies on small enough timescales
to make symmetrical lobes.
Option (c) tends to be less favoured at the moment for several reasons. Observations

4 Martin Hardcastle: Jets in nearby FRII radio galaxies
Figure 3. 8 GHz flux from the jet as a fraction of total 8 GHz flux, plotted against source
luminosity at 178 MHz, for 18 sources in the combined sample. Scale is logarithmic: power is
in 10 24 W Hz \Gamma1 sr \Gamma1 and assumes H0 = 50 km s \Gamma1 Mpc \Gamma1
,\Omega = 0. Some data points are taken
from Black (1992). As no attempt was made by Black to correct for background lobe flux, these
fractions represent upper limits.
Figure 4. The logarithmic power/linear size diagram for the combined sample. Power and
cosmology as in Figure 3; linear size in kpc. Sources which definitely have jets meeting the
criteria of Bridle & Perley are denoted by a filled circle.

Martin Hardcastle: Jets in nearby FRII radio galaxies 5
show many jets which can be traced from the nucleus to the hotspot; there are many
compact (and therefore short­lifetime) hotspots in lobes without visible counterjets; and
it is hard to imagine a physical mechanism by which the flipping of jet sidedness could
take place. Option (b) on its own seems to have problems explaining the enormous
predominance of one­sided jets: there seems to be no a priori reason why some radio
sources shouldn't be sitting in symmetrical environments. Further, the parsec­scale jets
in these sources, when detected, always point towards the brighter kiloparsec­scale jet,
which implies that any intrinsic asymmetry in jet emissivity has to begin on very small
scales and be maintained over very large distances. Option (a), on the other hand,
can't be the whole story; as I've already said, there are wide variations in the surface
brightnesses along the jets (and, where visible, the counterjets), together with (and
often associated with) significant jet bending, which must indicate some effects of the
environment on jet visibility. With our new data, we can try to quantify the extent
to which each of the three models apply by looking at limits placed on velocities by
observed jet­counterjet asymmetries, by comparing (where possible) the bending and
brightness variations in jets and counterjets, and by looking at the relations between jets
and hotspots (various authors have suggested that the brighter hotspot is found on the
same side as the jet, for example, although this trend was not apparent in the data of
Black et al.).
5. Jets --- wider implications
Historically, the jets in quasars and in FRI radio galaxies have tended to be better
studied because they are easier to detect. Models for jets in FRI sources are now quite
detailed, the most popular ones involving trans­ or sub­sonic turbulent flow, and have
been applied to a number of sources with success: because these sources are generally
nearby, 100­pc spatial resolution is possible with the VLA and excellent maps of the
jets can be obtained. It now seems very likely that relativistic velocities are present in
the inner parts of these jets, or at least the most powerful ones, and that the crucial
difference between FRIs and FRIIs is the rapid deceleration of the jets in the former
to trans­sonic velocities. The primary factor affecting this is clearly the jet power, as
indicated by the source luminosity, whence the result of Fanaroff & Riley (1974); but
intriguingly Owen (1993) has shown that the dividing line between FRI and FRII type is
also a function of the host galaxy luminosity, providing clear evidence for a link between
large­scale environment and jet properties.
Results are also now starting to emerge about the counterjets in quasars. In a preprint
now circulating, Bridle et al. (1994) present very detailed images of 12 quasars made with
the VLA at 4.9 GHz and discuss the properties of their jets and counterjet candidates.
From the point of view of our sample these results are important as they represent a
higher­luminosity sample of FRII objects (there are no FRII quasars with z ! 0:3 in
the LRL sample: the lobe­dominated objects at small angles to the line of sight in
unified schemes are probably the N­galaxies or broad­line radio galaxies). Jets in FRII
radio galaxies at higher redshifts are harder to detect because of the lack of spatial
resolution. By comparing these results with ours it should be possible to put more
stringent constraints both on the one­sidedness problem and on the effects of source
power on jet type.

6 Martin Hardcastle: Jets in nearby FRII radio galaxies
6. Summary
We now have an unrivalled database of high­resolution 8 GHz VLA maps of nearby
FRII radio galaxies. Jets are detected in up to 70% of the sample, although counterjets
are still rare. We expect to be able to put important constraints on the physics of jets in
these objects, which can then be compared with results on jets in quasars and FRI radio
galaxies.
The author is supported by a studentship from the PPARC. The NRAO Very Large
Array is operated by Associated Universities Inc. under contract with the National Sci­
ence Foundation.
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