1992MNRAS.259P..13P
Downloaded from https://academic.oup.com/mnras/article-abstract/259/1/13P/1040660 by California Institute of Technology user on 20 May 2020
Mon. Not. R. Astron. Soc. (1992)
259,
Short Communication,
13P-16P
A jet in the nucleus of the giant quasar 4C 7 4.26
T. J. Pearson,
1
K. M. Blundell,
2
J.M. Riley
2
and P. J. Warner
2
1
Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
2
Mu/lard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3
OHE
Accepted 1992 September 19. Received 1992 September 16
ABSTRACT
An
image of the nucleus of the giant quasar 4C 74.26 made using very long baseline
interferometry at a frequency of 5 GHz shows a one-sided, parsec-scale jet that is well
aligned with the 400-kpc jet seen in VLA images.
If
the jet asymmetry is due to
Doppler boosting, the axis of the source cannot lie close to the plane of the sky. The
radio spectrum of the nucleus, measured with the VLA, has a peak at about 8 GHz.
Key words:
galaxies: jets - quasars: general - quasars: individual: 4C 7 4.26 - radio
continuum: galaxies.
1 INTRODUCTION
The double-lobed radio source 4C 74.26 or 2043 + 74.9
(Riley et al. 1988) is identified with a 14.8-mag quasar of
redshift
z
=
0.104. The angular extent of 10 arcmin corre-
sponds to a projected linear size of 0.8
h-
1
Mpc (assuming
H
0
=
100
h
km
s-
1
Mpc-
1
,
q
0
=
0.5), making 4C 74.26 one of
the largest known sources associated with a quasar. An
image with 15-arcsec resolution made with the VLA at 1.46
GHz (Riley & Warner 1990) shows an unresolved nucleus of
230 mJy from which a single jet extends at least 150 arcsec
(200
h-
1
kpc) in PA 160° toward a bright hotspot in the
southern lobe. This hotspot is much brighter and more
compact than that in the northern lobe.
If
radio-selected quasars are an unbiased sample of a
randomly oriented population, the quasars with largest
projected size are not expected to show rapid superluminal
expansion. This is because apparent superluminal expansion
is attributed to relativistic motion along an axis close to the
observer's line of sight, and on average the axes of the largest
quasars will be almost perpendicular to the line of
sight.
The detection of superluminal motion in another large
quasar, 4C 34.47 (Barthel 1987; Barthel et al. 1989) was
thus a surprise, and has led to a re-examination of our
assumptions. Rapid superluminal expansion in the nucleus of
a large double-lobed quasar could indicate, for example, that
the motion in the nucleus is closer to the line of sight than
is•
the axis of the double-lobed source. An alternative and attrac-
tive possibility is that a powerful double-lobed radio source
appears as a quasar when its axis lies close to the line of sight
and as a radio galaxy otherwise (e.g. Readhead et al. 1978;
Peacock 1987; Scheuer 1987; Barthel 1989).
We report here first-epoch VIBI observations of the
nucleus of 4C 7 4.26 which show a one-sided parsec-scale jet
that is well aligned with the kiloparsec-scale jet. Future
observations may provide an expansion speed for features in
this jet. We have also used the VLA to measure the radio
spectrum of the nucleus.
2
OBSERVATIONS
2.1 VLBI observations
The observations were made on 1988 November 14, using
eight antennas of the European VIBI Network and the
United States VIBI Network.
1
The source 4C 74.26 was
observed for approximately 6 h. Two calibration sources
were also observed: 1739 + 52.2 in two 20-min scans and
2200+42.0 (BL Lacertae) in one 20-min scan. The observ-
ing frequency was 4991 MHz, the bandwidth 1.8 MHz, the
polarization left-circular, and the recording system Mark II.
The data were cross-correlated using the JPL/Caltech
Block-II Interferometry Processor at the California Institute
of Technology, and residual delays and fringe rates were
determined using a global fringe-fitting algorithm (Schwab
&
Cotton 1983) as implemented in the
AIPS
package of the
National Radio Astronomy Observatory (task
CALIB).
Amplitude calibration was based on measured system
temperatures and antenna gains, and on the observations of
1
MPifR, Effelsberg, Germany (diameter 100 m); Istituto di Radio-
astronomia, Medicina, Italy (32 m); Jodrell Bank, England (26 m);
Onsala Space Observatory, Onsala, Sweden (25 m); NEROC
Haystack Observatory, Westford, MA (37 m); NRAO, Green Bank,
WV (43 m); NRAO, Pie Town, NM (25 m, one antenna of the
partially completed Very Long Baseline Array); NRAO VLA,
· Socorro, NM (for part of the observations all 27 antennas were used
as a phased array; for the remainder a single 25-m antenna was
used). Observations at Westerbork (the Netherlands) and
OVRO
(California) were unsuccessful.
. ©
Royal Astronomical Society
•
Provided by the NASA Astrophysics Data System
1992MNRAS.259P..13P
Downloaded from https://academic.oup.com/mnras/article-abstract/259/1/13P/1040660 by California Institute of Technology user on 20 May 2020
14P
T.
J.
Pearson et al.
the calibration source 1739
+
52.2. The highly variable radio
source 1739
+
52.2 has been only barely resolved in
previous observations (Pearson
&
Readhead 1988), but self-
calibration of the present observations showed that it could
be modelled with an unresolved component of 1.1 Jy plus a
0.7-Jy halo of size 0.0015 arcsec
(FWHM
of equivalent
Gaussian brightness distribution) slightly displaced from the
unresolved component. The amplitude correction factors,
accurate to a few per
cent,
derived from the self-calibration
of 1739
+
52.2 were applied to the data for 2043
+
74.9. The
other calibration source, 2200
+
42.0, was heavily resolved
at the time of these observations. Self-calibration showed
that it was extended about 0.006 arcsec in PA 10°, consistent
with previous observations (Mutel et al. 1990 ).
An image of 2043
+
7 4.9 was obtained with an iterative
self-calibration algorithm (Pearson
&
Readhead 1984). In
each iteration, a model image was used to estimate complex
gain corrections for each antenna, and the corrected data
were used to estimate a new image for the next iteration. The
gain corrections were estimated by using an implementation
of the coRTEL algorithm (Cornwell & Wilkinson 1981); the
images were estimated from the corrected visibility data
using both CLEAN and a maximum entropy algorithm, which
provide alternative methods of enforcing the positivity
constraint.
(a)
5
0
Cl
........
0
G)
(I)
e
,g
0
.E
··.·
......,
0
G)
0
G)
>
:;:;
0
a;
a:::
-5
·
..
•
:
..
-10
I
5
0
-5
Relative R.A. (milliarcsec)
2.2 VLA observations
The core of 4C 74.26 was observed at 0.3, 1.4, 5, 8 and 15
GHz with the A-configuration of the VLA on 1988
November 16, to investigate its spectrum. One 15-min snap-
shot was made at each frequency; the flux density calibrator
was 3C 48. The data were calibrated at the VLA by standard
procedures. Subsequent reduction was carried out using the
AIPS package on a Sparcstation at
MRAO.
After initial
mapping and cleaning, the data were self-calibrated for
phase. The core flux densities were measured from these
maps by fitting a Gaussian, zero offset and slope with the AIPS
task1Mm.
3 RESULTS
The flux density of the nucleus of 4C 74.26 detected in the
VLBI observation was 0.30
±
0.03 Jy, and it was clear from
the visibilities that
- 0.1 Jy was due to an unresolved com-
ponent ( <0.5 mas), while the remainder was due to an
elongated feature. Fig. 1 shows two images made from the
same data. The first, a conventional 'clean' image, is
convolved with an elliptical Gaussian representing the
effective synthesized beam. The second is a maximum
entropy image, which can be regarded as a deconvolution of
(b)
5
0
0
©>
0
()
<>
C)
-5
<>
-10
5
0
-5
Relative R.A.
(milliarcsec)
Figure
1.
VLBI images of the nucleus of 4C 74.26 at 4.99 GHz. The linear scale is 1 mas= 1.3
h-
1
pc. {a) Conventional 'clean' image. The
image has been convolved with an elliptical Gaussian restoring beam of FWHM 1.5 x 0.75 mas
2
,
with the major axis in PA
9°
(hatched ellipse).
The logarithmically spaced contour levels are
±
1,
±
2, 4, 8, 16, 32 and 64 per cent of the peak, which has brightness 0.14 Jy per
beam.
The
lowest contour is at 2.3 times therms noise level {0.6 mJy per beam). The spurious low-level extension to the north-east is due to a residual
calibration error. (b) Maximum entropy image made from the same visibility data. The contour levels are 0.5, 1, 2, 4, 8, 16, 32 and 64 per cent
-
of the maximum. The detached, low-brightness features are almost certainly due to residual calibration errors.
If
this image is convolved with
the elliptical Gaussian used
in
(a), a very similar image results.
©
Royal Astronomical Society
•
Provided by the NASA Astrophysics Data System
1992MNRAS.259P..13P
Downloaded from https://academic.oup.com/mnras/article-abstract/259/1/13P/1040660 by California Institute of Technology user on 20 May 2020
the clean image. The images show that: (i) the emission
region is highly elongated in PA 160° (B1950.0), the same
position angle as that of the inner part of the large-scale jet
(Riley & Warner 1990 ); it extends for 4.5 mas ( 6
h-
1
pc) in
this direction and is unresolved (
<
0.5 mas,
<
0.6
h-
1
pc) in
the perpendicular direction;
(ii)
the brightest point is close to
the northern end, and the brightness decreases smoothly
toward the south. Such elongated brightness features are
commonly called 'jets' (Bridle & Perley 1984 ); in this source,
the parsec-scale jet is presumably connected to the kilopar-
sec-scale jet seen in the VLA image, and it is likely that its
southward extent in the VLBI image is limited by the sensi-
tivity of the observations.
The flux density of the core measured on the shortest
VLBI baselines agrees very well with that obtained from the
VLA map at 4885 MHz (328 mJy), and all the flux detected
with the VLA is accounted for on the VLBI maps shown in
Fig. 1. The core flux density is variable at 5 GHz (Riley et al.
1988) but, as the VLBI and VLA observations were made
only two days apart, this should present no problems when
comparing these values. Only a very small amount of the
VLA core flux (
<
20 mJy) can arise from scales larger than 4
mas. Despite this, however, limits on the surface brightness of
any continuation of the parsec-scale jet are still considerably
higher than that of the kiloparsec-scale jet (Riley & Warner
1990).
It is not clear whether there is a counter-jet directed
towards the northern lobe.
If
the centre of activity is assumed
to be at the brightest point then there is no evidence for a
counter-jet, and a lower limit of
- 50: 1 can be placed on the
jet/counter-jet brightness ratio close to the centre.
In
order to
confirm, though, that the brightest point does represent the
centre of activity, observations at another frequency are
required to show that it has the flat spectrum characteristic of
such 'cores'. The integrated flux densities of the core
obtained from the VLA maps were: 70 ± 6 mJy (0.327 GHz);
184±3 mJy (1.46 GHz); 328±1 mJy (4.88 GHz); 333±1
mJy (8.4 GHz); 306 ± 2 mJy (14.9 GHz). Thus the integrated
spectrum of the core (Fig. 2 and Riley et al. 1988) shows that
some part of the core has an inverted spectrum up to about
10 GHz and has emission in the millimetre region.
In
this
context, however, it is clear from Fig. 2 that, since the parsec-
scale jet contributes about two-thirds of the flux at 5 GHz, all
or part of it must be self-absorbed and have a flat spectrum
below this frequency. The question of the true 'core' will only
be answered by higher frequency VLBI observations. Thus
the VLBI observations do not resolve the question of
whether there is a northern jet at present, or whether it is
hidden by Doppler beaming.
If
it is assumed that the
asymmetry is entirely due to Doppler beaming with jet flow
speed
{3c
at an angle to the line of sight
0,
the brightness ratio
of
>
50: 1 requires that
{3
cos
0
>
0.65 and hence that
0
:S
49°
(Bridle & Perley 1984; the jet spectral index has been taken
as
a=
-0.5,
Sex:
va).
This suggests that the axis of the double
radio source is not close to the plane of the sky and implies
that the deprojected size of the parsec-scale jet is
~
8
h
-
1
pc
and that the whole source is
~
1.1
h-
1
Mpc in extent.
Unfortunately, the parsec-scale jet is smooth, and shows
no brightness peaks or 'knots' that could be monitored to
measure an apparent jet speed. The total flux density of the
core at 4995 MHz decreased from 0.42 to 0.31 Jy between
1986 and 1988 (though from the present VLA observations
A jet in the nucleus of4C 74.26
15P
1
0.01
0.1
1
10
100
1000
Frequency
(GHz)
Figure 2.
Spectrum of the nucleus of 4C 74.26. The data from 0.3
to 15 GHz were measured with the VLA on 1988 November 16
(see Section 2.2). The point at 270 GHz is from Riley et al. {1988).
there is no evidence for further changes), and in other
sources flux outbursts are often associated with the ejection
of knots into the jet. Knots might also be detectable in
observations with higher dynamic range.
4 CONCLUSIONS
The nucleus of 4C 7 4.26 contains a radio jet of length at
least 6
h-
1
pc which is well aligned with the 200
h-
1
kpc jet
seen in a VLA image. There is at present no evidence for a
counter-jet.
If
the asymmetry of the parsec-scale jet is
attributed to Doppler boosting, the axis of the source must
lie
:$
49° from the line of sight. Unfortunately, there are no
clear 'features' such as discrete brightness peaks that could
be used to estimate an expansion velocity. Observations with
higher resolution or higher surface brightness sensitivity
will
be needed to detect such features.
ACKNOWLEDGMENTS
We are grateful to the European and United States VLBI
networks for scheduling the observations reported here, and
we thank the staffs at the telescopes and the correlator for
their expert assistance. We thank Dr P. D. Barthel for
comments on the manuscript, and Dr S.
F.
Gull for providing
the maximum entropy imaging program. The work was
supported by the National Science Foundation through its
support of the US VLBI network and by grant AST 88-
14554 to the Owens Valley Radio Observatory. TJP thanks
the Netherlands Organization for Scientific Research for a
visiting scholarship which supported part of this work. The
National Radio Astronomy Observatory is operated
by
Associated Universities, Inc., under cooperative agreement
with the National Science Foundation.
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P. D.,
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K.,
Preuss
E., 1989, ApJ, 336,601
Bridle A.H., Perley R. A., 1984, ARA&A, 22, 319
Cornwell T. J., Wilkinson P. N., 1981, MNRAS, 196, 1067
Mutel R.
L.,
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©
Royal Astronomical Society • Provided by the NASA Astrophysics Data System