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The distant DRAGNs survey
By J. Dun c a n L a w­G r een
e­mail: dlg@jb.man.ac.uk
Nuffield Radio Astronomy Laboratories, Jodrell Bank, Lower Withington, Macclesfield,
Cheshire SK11 9DL, UNITED KINGDOM
The Distant DRAGNs Survey (DDS) is a long­term project to form high­resolution radio images
with MERLIN of a sample of 38 high­z DRAGNs (the radio sources associated with radio galaxies
and radio­loud quasars) at two levels of flux density. The objective of the project is to study the
cosmological evolution of DRAGNs by comparison of this sample with low P=z samples. This
paper briefly discusses some of the processes which may be affecting high­z DRAGNs, and how
they may be investigated with sub­kpc radio imaging. The current state of DDS observations
is reported. A 1.4/1.6­GHz MERLIN image of 3C 239 (z = 1:781) is presented and discussed.
1. Introduction
1.1. What is a DRAGN?
DRAGNs, or Double Radio Sources Associated with Galactic Nuclei, are the extended
radio sources associated with radio galaxies and radio­loud quasars (Leahy (1993)). They
have an accepted qualitative explanation in the beam model of Scheuer (1974) and Bland­
ford & Rees (1974), where mass, momentum and magnetic flux are continually ejected by
the nucleus (probably a – 10 8 M fi black hole), conducted through a narrow collimated
beam, and supplied to the hotspots.
1.2. Cosmological evolution
The strong cosmological evolution of powerful DRAGNs first came to light over thirty
years ago, with the first systematic studies of the radio source counts N (S). It was
found that the source counts increased more strongly with decreasing flux density than
predicted for a non­evolving Euclidean Universe (Ryle & Clarke (1961)). It is now known
that the comoving volume density of DRAGNs at z ¸ 1 is factors of 1000 to 10000
higher than at the present epoch (Condon (1989), Dunlop & Peacock (1990)). There is
considerable evidence that a number of other source parameters undergo cosmological
evolution (see Section 2).
In order to investigate DRAGN cosmological evolution in greater detail, we have chosen
to image in detail small samples in opposite corners of the P \Gamma z plane, on the basis that
studying such extreme cases should make population changes easier to see. Some of
these changes may reveal information about the interaction between the jet and the
intergalactic medium, and may also help us to trace changes in the environments of
DRAGNs with cosmological epoch.
The nearest FRII DRAGNs (z ' 0:1) have already been well mapped with the VLA
(e.g. Leahy & Perley (1991), Black et al. (1992)). We have selected a comparison flux­
limited sample of 28 radio galaxies and 10 quasars at z ? 1:5. The Distant DRAGNs Sur­
vey (DDS) is a long­term project --- in collaboration with: P. Alexander, J.R. Allington­
Smith, S.A. Eales, J.P. Leahy and S.G. Rawlings --- to image each of these high­redshift
DRAGNs at two or more frequencies with MERLIN. Distant DRAGNs typically have
small angular sizes (both due to their distance, and to pronounced linear size evolution),
and MERLIN is the only instrument able to achieve sub­kpc linear resolution at the low
frequencies required to detect steep­spectrum extended structure.
1

2 J.D.B. Law­Green: The distant DRAGNs survey
Section 2 describes some of the scientific objectives of the Distant DRAGNs Survey.
Section 3 describes the DDS sample. Section 4 describes the current state of observations.
Section 5 presents some initial results of DDS observations at 1.4 GHz.
2. Scientific goals
This section discusses a number of predictions regarding high­z DRAGNs which can
be investigated with the aid of sub­kpc radio imaging:
ffl Structure of hotspots: Jets in FRII DRAGNs are ¸ 1 kpc across, and simulations
indicate that the hotspot terminal shock structures are a few times smaller than this.
How does the hotspot structure of distant, powerful DRAGNs compare with that of
nearby objects? Do they follow the ``primary plus secondary component'' model of Laing
(1989)? The nature of any changes will depend on whether the power of the jet is
primarily a function of speed or density.
ffl Alignment effect: The UV, optical and IR emission from high­z radio galaxies tends
to be extended and aligned with the axis of the radio source (McCarthy et al. (1987).
These galaxies show blue optical/IR colours, suggesting that the interaction of the radio
jet with the ISM is forming aligned populations of young stars (e.g. Chambers et al.
(1988)). Other possibilities are scattering of light from a hidden quasar, supported
by the detection of optical polarisation (di Serego Aligheri et al. (1989)), or inverse
Compton scattering of CMB photons (Daly (1992a), Daly (1992b)). Comparison of
radio and optical images on sub­kpc scales provides the most straightforward method of
distinguishing between the possible processes (Miley (1992)).
ffl Gravitational Lensing: Many distant 3CR sources may only appear in the catalogue
because they are gravitationally amplified by lensing (e.g. Hammer & Le F`evre (1990)).
Lensing conserves surface brightness, so strong amplification implies strong distortion,
with a reasonable change of multiple imaging or ring formation. The linear, polarised
structures of jets are also suitable as probes for lensing objects (Kronberg & Dyer (1993)).
Detection of such lensing events can place limits on the inhomogeneity of the Universe
(Blandford & Jaroszynsk (1981)).
ffl Radio Bridges: These diffuse areas of low surface brightness emission between the
hotspots are thought to represent the accumulation of relativistic particles accelerated
though the hotspots over the source lifetime, and now forming a low­density ``cocoon''
around the source. The brightness of radio bridges falls rapidly with increasing source
power and/or redshift (Jenkins & McEllin (1977)), possibly due to inverse Compton
scattering by CMB photons. The axial ratio of DRAGNs appears to increase with source
power (Leahy et al. (1987), hereafter LMS), but selection against small/distant sources
mean that a correlation between axial ratio and linear size may be mimicking this effect.
LMS's sample contains only two sources at z ? 1:5 and the bridge structures of high­z
DRAGNs remain poorly studied.
3. Sample selection
Our sample of distant DRAGNs is drawn from the three low­frequency surveys which
have most complete optical identification and redshift data; the 3CR catalogue (Spinrad
et al. (1985)), the 1 Jy B2 sample of Allington­Smith (1982) and the overlapping 2 Jy
6C sample (Eales (1985)). Use of low­frequency surveys selects against core­dominated
DRAGNs, which, according to unified schemes (e.g. Barthel (1989)) are aligned pref­
erentially towards the line of sight, or else are somewhat different from lobe­dominated
DRAGNs.

J.D.B. Law­Green: The distant DRAGNs survey 3
3CR 6C/B2
Source IAU Name ID z `( 00 ) Source IAU Name ID z `( 00 )
3C9 0017+154 Q 2.012 13.5 4C35.20 0824+355 Q 2.249 2.0
3C68.2 0231+313 G 1.575 22.3 0901+35 G 1.91 3.0
3C191 0802+103 Q 1.9523 4.9 0902+343 G 3.395 4.5
3C205 0835+580 Q 1.534 16.0 4C39.24 0905+39 G * 113.0
3C225A 0939+140 G 1.56 4.9 0930+38 G 2.40 4.7
3C239 1008+467 G 1.781 11.1 0955+396 Q 2.94 2.7
3C241 1019+222 G 1.617 0.9 1016+36 G * 18.1
3C256 1118+237 G 1.819 4.2 4C39.33 1042+39 G 1.76 9.0
3C257 1120+057 G 2.474 13.2 1045+34 G * 17.5
3C270.1 1218+339 Q 1.519 12.0 1056+396 G 2.171 14.9
3C280.1 1258+404 Q 1.659 19.8 1106+38 G * 0.7
3C294 1404+344 G 1.779 14.5 4C34.34 1113+34 G * 16.0
3C322 1533+557 G 1.681 33.0 1132+37 G 2.28 1.6
3C326.1 1553+202 G 1.825 6.7 1134+36 G 2.12 16.5
3C432 2120+168 Q 1.805 14.7 4C35.26 1141+354 G 1.781 11.6
3C454.1 2248+712 G 1.841 1.6 1159+368 G * 1.6
3C454 2249+185 Q 1.757 1.0 4C37.33 1204+37 G * 60.0
3C470 2356+438 G 1.653 23.9 1212+38A y G * 0.6
1225+36 Q 1.975 ! 0:07
4C39.37 1232+397 G 3.22 7.7
1301+35 G * ! 0:23
Table 1. The Distant DRAGNs Survey sample. ID indicates the optical identification type (Q:
Quasar, G: Galaxy). y: Not yet confirmed as a sample member. *: Redshift will be presented
in Rawlings et al. (in preparation).
3.1. 3CR sample
The 3CR catalogue essentially covers the brightest radio sources in the northern hemi­
sphere away from the galactic plane (jbj ? 10 ffi ). Typical 3CR members have S 178 ¸ 12
Jy. All 254 DRAGNs in 3CR are now optically identified, with all but two having spec­
troscopic redshifts (H. Spinrad, priv. comm.). Imposing a redshift cutoff of z ? 1:5 gives
a subsample of 11 galaxies and 7 quasars (Table 1).
3.2. 6C/B2 sample
The conjoining of the 6C and B2 samples (referred to here as ``6C/B2'') contains 82
DRAGNs, typically six times fainter than 3CR. The 6C/B2 DRAGNs are now well
mapped at arcsecond resolution with the VLA (Naundorf (1992); Law­Green et al., in
preparation). Optical/IR identifications have been found for all members of both samples
except for one 6C source obscured by a star (Eales et al., in preparation). Spectroscopic
redshifts are available for 75 of the 82 sources; the remainder are likely to be at z Ÿ 1
(Rawlings et al., in preparation). 6C/B2 now has the most complete redshift fraction of
any flux­limited sample fainter than 3C.
All the objects in the sample with z ? 1:7 have been selected giving a subsample of 14
galaxies and 3 quasars (plus one possible sample member, 6C 1212+38A, see Table 1).

4 J.D.B. Law­Green: The distant DRAGNs survey
4. Observations
4.1. Radio observations
The Distant DRAGNs Survey has required the gathering of a uniquely comprehensive
database of MERLIN observations at three frequencies (408 MHz, 1.4 GHz, 5 GHz),
totalling over 600 hours of observing time. The observational goal of the DDS is to
obtain high­quality MERLIN images of each DRAGN at two or more frequencies. The
higher frequency observations will give 40--140 beamwidths across each source, while the
lower frequency will bring out faint steep­spectrum emission and allow spectral index
mapping. VLA observations of sample members by ourselves and others (Law­Green et
al., in preparation) will also be used where needed to fill in short uv spacings and allow
valid spectral comparisons.
MFS: Several DRAGNs are sufficiently large that they cannot be imaged accurately
using MERLIN at a single observing frequency. We have therefore made use of Multi­
Frequency Synthesis (MFS) whereby the observing frequency is changed within a ¸ 20%
band on short timescales (typically ¸ 10 min). This has the benefit that u and v are
changed by the ratio of the observing wavelengths, so that a single physical baseline
sweeps out multiple tracks in the uv plane. The drawback is that spectral effects must
be accounted for, but a simple overall correction factor can often be applied (Conway et
al. (1990)). Reduction of MFS data is also time­consuming, since each frequency must
be calibrated separately.
The current state of observations is as follows:
ffl P­Band (408 MHz): 19 targets (total 220 hours). Observations underway.
ffl L­Band (1.4--1.6 GHz): 10 targets (total 228 hours). Data reduction complete.
ffl C­Band (5 GHz): 12 targets (total 195 hours). Proposal submitted.
4.2. Optical/IR observations
We are presently gathering a comprehensive database on our high­redshift sample at
optical and infrared wavelengths. Current efforts include:­
ffl UKIRT: Deep K­band images have been made of a subset of high­redshift 6C radio
galaxies, to see whether they follow the same K \Gamma z relationship as 3C radio galaxies,
and whether their IR and radio structures align (Eales et al., in preparation).
ffl HST: A proposal has been submitted to image 15 of our 6C radio galaxies with the
Hubble Space Telescope.
5. Results
For reasons of limited space, I am not able to present the full scope of the DDS
results obtained so far, but will instead show one image which is representative of the
image resolution and quality which can be obtained with MERLIN using the techniques
discussed above.
5.1. 3C 239
Figure 1 shows a uniformly weighted image of 3C 239 produced using frequency­switching
MFS at 1420 MHz and 1658 MHz.
3C 239 is an FRII radio galaxy at z = 1:781. Note that the brighter eastern lobe
is extended perpendicular to the source axis; the lobe spectral index steepens in this
direction away from the double hotspot (Liu et al. (1992)). The optical field of 3C
239 is crowded; it may lie in a distant cluster, possibly gravitationally amplified by a
foreground cluster (Hammer & Le F`evre (1990)). An R = 21:8 galaxy (``b'') lies only

J.D.B. Law­Green: The distant DRAGNs survey 5
Figure 1. MERLIN image of 3C 239, using Multi­Frequency Synthesis at 1420 MHz and
1658 MHz. The uv data were uniformly weighted, and subsequently reweighted for telescope
sensitivities. The beam size is 0:145 00 .
1:5 00 from the eastern radio lobe. The eastern lobe is significantly more depolarized
than the western lobe (Liu & Pooley (1991)). This suggests that galaxy b might be a
foreground object amplifying the radio emission from the eastern lobe by gravitational
lensing. Alternatively, galaxy b may be located in the same cluster as 3C 239; the radio
morphology may be caused by the interruption of the radio jet by b's halo. This latter
suggestion is somewhat supported by b's unusual morphology in an R­band HST image
(P. Best, private communication).
6. Conclusions
The Distant DRAGNs Survey is the first major observational programme to image a
flux­limited sample of radio galaxies and radio­loud quasars at high redshift with sub­kpc
linear resolution. While observations are still under way, the quality of images obtained
so far indicates that the project has great promise for determining the underlying physics
of DRAGNs, and probing the environments of radio sources at high redshifts.
DL­G acknowledges receipt of a Postgraduate Studentship from PPARC. MERLIN is
a national facility operated by the University of Manchester on behalf of PPARC.
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