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Astron. Astrophys. 329, 809-820 (1998)

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2. Observations and analyses

2.1. Sample selection

Radio, optical and infrared observations of the 558 sources in the MRC/1Jy sample have resulted in the discovery of about 40 sources at z [FORMULA] 2 (McCarthy et al. 1997). The redshifts of 19 of these have been confirmed spectroscopically while the rest have been estimated from the well known relationship between infrared K-band magnitude and redshift (Lilly 1989; McCarthy 1993). The optical identification of the sample is complete to 95 per cent (McCarthy et al. 1996) while redshift information (spectroscopic or from K-magnitudes) is available for 80 percent of the galaxies. We do not think that the incompleteness of the spectroscopy has biased the HRRG sample seriously since the incompleteness is essentially due to paucity of observing time at some right ascensions. There were 17 galaxies known in 1993 with spectroscopic z [FORMULA] 2. High resolution multifrequency observations of 15 of them were undertaken in 1993 using the VLA radio telescope. The other two sources could not be observed due to constraints imposed by the range of LST allocated to our project. These 15 radio galaxies are unlikely to be significantly biased in any respect and are expected to be representative of the z [FORMULA] 2 galaxies in the MRC/1Jy sample.

2.2. Observations

The details of the observations of the 15 HRRGs are listed in Table 1. The sources were observed for 10-30 minutes in 3-5 bands, each band consisting of 2 frequencies of 50 MHz bandwidth. The observations for each source were split into several (2-4) shorter sessions at different hour angles to improve the u-v coverage. The flux calibration was done from observations of 3C 48 and 3C 286 on the scale of Baars et al. (1977) together with the recent corrections determined by Perley in AIPS 1 (see the HELP files on AIPS version 15JULY95). The phase calibration and determination of the instrumental polarisation was done using 0237-233, 1143-245 and 2331-240. The PPA was calibrated using 3C 138 (-9[FORMULA] at 1.425 GHz and -12[FORMULA] at higher frequencies) and 3C 286 (33[FORMULA] at all frequencies).


Table 1. Polarimetric observations of MRC/1Jy high redshift radio galaxies (z [FORMULA] 2) using the VLA. The mean frequencies are given within angular brackets. The project codes are VLA AA155 (code A) and VLA AC374 (code C). The bandwidth is 50 MHz at all frequencies.

2.3. Data reduction

The data were analysed in a standard manner using the AIPS package. After the initial flux density, phase and polarisation calibration, the visibility data were passed through 3-4 rounds of SELF-CAL (phase only) to obtain an rms noise of [FORMULA] 0.05-0.15 mJy/bm in the final images.

For each source, the best quality Stokes I image was obtained by the procedure described above. The same data was used to obtain images in Stokes Q, U and V. This was done independently at both frequencies in each band. Since extended extragalactic sources are not expected to have any circular polarisation, the V images, expected to be pure noise images, were used to estimate the quality of the calibration procedure. At 8.4 GHz, in addition to the high resolution images, lower resolution images similar to those at 1.4 and 4.8 GHz were made by weighting down the long baselines during analysis. This was necessary since polarisation properties are known to be resolution dependent. Similarly, matched higher resolution images were also obtained at 4.71, 8.21 and 8.44 GHz.

UV-tapering of the 4.71 and 8.21 GHz data to obtain lower resolution images similar to those at 1.43 and 4.86 GHz did not prove successful. The short duration of the 4.71 and 8.21 GHz observations in the A array configuration resulted in very few short baselines and the weighting down of the much more numerous long baselines reduced drastically the effective number of baselines leading to excessively noisy images.

The Stokes images were used to obtain maps of total polarisation (P = [FORMULA]) and the PPA ([FORMULA]) by combining the images appropriately, pixel by pixel. The total polarisation images were corrected for the Ricean bias in the noise statistics.

2.4. Rotation measure calculation

The resolutions at 1.43 and 4.86 GHz (3-4") are similar to the sizes of the individual radio lobes and hence inadequate for studying the variation of polarisation properties across the lobes. So the PPA data presented here as well as the RM calculated from them are (polarized flux weighted) average values for the entire lobe. The higher resolution images at 4.71 and 8.21 were from shorter duration observations at fewer hour angles and it was found that the measurements were not very reliable at faint polarised flux levels. So the PPA measurements in these images were also confined to the area around the hotspots, which had high signal-to-noise ratio.

The PPA was obtained by averaging the values within a box (9-25 pixels) centred around the peak of the polarised emission for each radio lobe. The shift in the peak positions between different frequencies was found to be [FORMULA] 1 pixel (beam width 5-8 pixels along each axis). The radio contour maps of 0015-229, which are typical of the sample, are shown in Fig. 1. The radio polarisation maps of all the sources are being published elsewhere (Athreya et al. 1997b) together with all the other images and data.

[FIGURE] Fig. 1. Radio contour images of 0015-229 (z = 2.01) with polarisation vectors superposed. The co-ordinates of the centre, the beam size and the scale for polarised flux, which are common for both the images, are listed below them. The frequency and the contour scale factor (CLev) are listed above the images. The contour levels are Clev [FORMULA] [-2, -1, 1, 2, 4, 8, 16, [FORMULA] ]. The superposed vectors indicate the direction and magnitude of the fractional polarisation.

A 5 [FORMULA] detection threshold was considered sufficient for the polarised flux. RMs were calculated only for those radio lobes which were detected in at least 2 different bands (i.e. 3-4 frequencies). Most of the detections were at the level of [FORMULA] 10 [FORMULA] and the errors on the PPA values are 5-10[FORMULA] (including the 2-3[FORMULA] error in the primary PPA calibrator).

Lack of resolution at lower frequencies forced us to observe at frequencies [FORMULA] 1.4 GHz. The rather small difference in the rest frame [FORMULA] among the 3 bands (particularly between 4.71 and 8.4 GHz) resulted in considerable errors on the intrinsic RM values despite the large number of frequencies of observation.

The (n [FORMULA] 180[FORMULA]) ambiguity of the PPA was resolved in most cases by examining the PPA change between closely spaced frequencies (actually closely spaced in [FORMULA]) - 1.3851 & 1.4649 GHz, 4.5351 & 4.8851 GHz, 4.86 & 8.44 GHz bands. The 180[FORMULA] ambiguity in the PPA implied that we were incapable of measuring observed RMs in excess of [FORMULA] 1200 rad m-2 (or [FORMULA] 630 rad m-2 in objects without the 4.71 GHz band observations), which corresponds to intrinsic RMs at z [FORMULA] 2 of [FORMULA] 12000 rad m-2 (or [FORMULA] 6300 rad m-2). A straight line was then fit to all the data points to obtain a RM consistent with the value obtained from the close pairs. The smallest value of the RM was found to provide the best fit to the data of all radio lobes excepting two (explained in the notes on individual sources). The change in PPA with frequency can also be caused by many independent polarised components within the telescope resolution. Multiple components with very different spectral indices and fractional polarisation may mimic Faraday rotation with the PPA of different components dominating at different frequencies. However, a linear relationship between the PPA and [FORMULA] is the signature of Faraday rotation.

The PPA from low and high resolution observations were combined to calculate the RM in several sources (indicated in Table 2). However, it was first checked that the measured PPA values from images of different resolutions but at similar frequencies were consistent with each other ([FORMULA] 4.8 GHz images from the A and CnB configurations, untapered and tapered 8.44 GHz images from the BnA config. and untapered 8.21 GHz images from A config.).


Table 2. Observed rotation measures in the lobes of MRC/1Jy galaxies at z [FORMULA] 2. The columns are redshift (z), remarks (R), observed RM (RMobs), flux density at 4.86 GHz (S4.86) and the fractional polarisation (%-pol) within parantheses, residuals of the linear fit ([FORMULA]: [FORMULA] the mean of the residuals and Mx the maximum value), reduced [FORMULA] and number of points (N) of the fit. The remarks are `a' (an additional 180[FORMULA] rotation given to the PPA of 1.425 GHz), `l' (low resolution images: 3-4 arcseca) and `h' (high resolution images (0.5-0.7 arcsec).

This may be verified in Fig. 2 from the plots for 0406-244 P, 1106-258, 1138-262, 1324-262, 2139-292 (these being the only sources with a mixture of PPA from different resolutions). In each case, each of the clusters of points at [FORMULA] 1.3 and 3.8, consists of 3-4 data points obtained from images at 2 different resolutions. The consistency between the values from different resolutions suggests that either the polarised flux is dominated by the emission from the immediate vicinity of the hotspot (2-5 kpc) or the same Faraday screen covers all parts of the radio lobe contributing substantially to the observed polarised flux.

[FIGURE] Fig. 2. Polarisation position angle in degrees (y-axis) v/s [FORMULA] in 10-3 m-2 (x-axis) for the lobes of MRC/1Jy galaxies at z [FORMULA] 2. The F or P following the source names indicate the Following and Preceding lobes, respectively. Two plots are shown for each of 0156-252 F and 0406-244 F, with an additional 180[FORMULA] rotation.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997