The distant DRAGNs survey

J. Duncan Law-Green
e-mail: dlg@jb.man.ac.uk

Nuffield Radio Astronomy Laboratories, Jodrell Bank, Lower Withington, Macclesfield, Cheshire SK11 9DL, UNITED KINGDOM

Abstract:

Contents

• 1. Introduction
  • 1.1. What is a DRAGN?
  • 1.2. Cosmological evolution
• 2. Scientific goals
• 3. Sample selection
  • 3.1. 3CR sample
  • 3.2. 6C/B2 sample
• 4. Observations
  • 4.1. Radio observations
  • 4.2. Optical/IR observations
• 5. Results
  • 5.1. 3C 239
• 6. Conclusions
• Acknowledgments
• References

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 radio-loud quasars (Leahy (1993)). They have an accepted qualitative explanation in the beam model of Scheuer (1974) and Blandford & Rees (1974), where mass, momentum and magnetic flux are continually ejected by the nucleus (probably a 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 . It was found that the source counts increased more strongly with decreasing flux density than predicted for a non-evolving Euclidean Universe (Ryle & Clarke (1961)). It is now known that the comoving volume density of DRAGNs at 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 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 () 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 . The Distant DRAGNs Survey (DDS) is a long-term 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 high-redshift 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 sub-kpc linear resolution at the low frequencies required to detect steep-spectrum extended structure.

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 high- DRAGNs which can be investigated with the aid of sub-kpc radio imaging:

• Structure of hotspots: Jets in FRII DRAGNs are 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.
• Alignment effect: The UV, optical and IR emission from high- 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 sub-kpc scales provides the most straightforward method of distinguishing between the possible processes (Miley (1992)).
• Gravitational Lensing: Many distant 3CR sources may only appear in the catalogue because they are gravitationally amplified by lensing (e.g. Hammer & Le Fèvre (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)).
• 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 low-density ``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 and the bridge structures of high- DRAGNs remain poorly studied.

3. Sample selection

Our sample of distant DRAGNs is drawn from the three low-frequency surveys which have most complete optical identification and redshift data; the 3CR catalogue (Spinrad et al. (1985)), the 1 Jy B2 sample of Allington-Smith (1982) and the overlapping 2 Jy 6C sample (Eales (1985)). Use of low-frequency surveys selects against core-dominated DRAGNs, which, according to unified schemes (e.g. Barthel (1989)) are aligned preferentially towards the line of sight, or else are somewhat different from lobe-dominated DRAGNs.

3.1. 3CR sample

The 3CR catalogue essentially covers the brightest radio sources in the northern hemisphere away from the galactic plane (). Typical 3CR members have Jy. All 254 DRAGNs in 3CR are now optically identified, with all but two having spectroscopic redshifts (H. Spinrad, priv. comm.). Imposing a redshift cutoff of 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); Law-Green 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 (Rawlings et al., in preparation). 6C/B2 now has the most complete redshift fraction of any flux-limited sample fainter than 3C.

All the objects in the sample with have been selected giving a subsample of 14 galaxies and 3 quasars (plus one possible sample member, 6C 1212+38A, see Table 1).

  
Table 1: The Distant DRAGNs Survey sample. ID indicates the optical identification type (Q: Quasar, G: Galaxy). : Not yet confirmed as a sample member. *: Redshift will be presented in Rawlings et al. (in preparation).

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 high-quality 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 steep-spectrum emission and allow spectral index mapping. VLA observations of sample members by ourselves and others (Law-Green 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 band on short timescales (typically 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 time-consuming, since each frequency must be calibrated separately.

The current state of observations is as follows:

• P-Band (408 MHz): 19 targets (total 220 hours). Observations underway.
• L-Band (1.4-1.6 GHz): 10 targets (total 228 hours). Data reduction complete.
• C-Band (5 GHz): 12 targets (total 195 hours). Proposal submitted.

4.2. Optical/IR observations

We are presently gathering a comprehensive database on our high-redshift sample at optical and infrared wavelengths. Current efforts include:-

• UKIRT: Deep K-band images have been made of a subset of high-redshift 6C radio galaxies, to see whether they follow the same relationship as 3C radio galaxies, and whether their IR and radio structures align (Eales et al., in preparation).
• 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: MERLIN image of 3C 239, using Multi-Frequency Synthesis at 1420 MHz and 1658 MHz. The uv data were uniformly weighted, and subsequently reweighted for telescope sensitivities. The beam size is .

Figure 1 shows a uniformly weighted image of 3C 239 produced using frequency-switching MFS at 1420 MHz and 1658 MHz.

3C 239 is an FRII radio galaxy at . 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 Fèvre (1990)). An galaxy (``b'') lies only 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 -band HST image (P. Best, private communication).

6. Conclusions

The Distant DRAGNs Survey is the first major observational programme to image a flux-limited sample of radio galaxies and radio-loud quasars at high redshift with sub-kpc 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.

Acknowledgments

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