Astron. Astrophys. 329, 399-408 (1998)

2. Observations and data reduction

We have observed seven objects with ; four radio galaxies and three radio-loud quasars. Table 1 shows the target details. The radio galaxies TX 0211-122 and MG 1019+0535 were observed because their UV spectra are suggestive of extinction by dust (van Ojik et al. 1994; Dey et al. 1995), MRC 0943-242 because of the the presence of a large amount of neutral hydrogen surrounding the host galaxy (Röttgering et al. 1995) and 1243+036 because of its very high redshift (van Ojik et al. 1996). Among the quasars, we recall that PKS 1251-407 is, to date, the most distant known radio-loud quasar (Shaver, Wall & Kellermann 1996). MRC 1043-291 (Kapahi et al. 1997) and PKS 1354-107 (Shaver et al. 1997, in preparation), were selected because of their convenient observability during the observing run.

Table 1. Summary of the observations.

2.1. SEST observations

The observations with the 15-m SEST (Swedish-ESO Submillimetre Telescope) were made at La Silla, Chile, during 1995 July and 1996 May. The telescope was equipped with a ³ He-cooled 1.3-mm bolometer, with a central frequency of 250 GHz and a bandwidth of around 50 GHz. The beamsize is about 24 (FWHM) and the typical sensitivity is 200 mJy s . The observations were carried out during nights with rather low opacities. The opacity was checked by several skydips taken during the nights (i.e. the telescope was moved to six different elevations where the bolometer integrated for 10 s and a calculation of the zenith opacity was made). The pointing was checked using quasars as astrometric reference sources and the absolute flux calibration was achieved by observing the planet Uranus. We estimate that the flux calibration has a typical uncertainty of 20%. The observations were made in beam-switching mode with a beam throw of 70, and the data were reduced according to method described by Andreani (1994) (see also Andreani et al. 1993). We have found a linear decrease of the standard deviation with , indicating that the observing conditions were stable and limited by the sky noise.

2.2. IRAM observations

Observations with the IRAM 30-m antenna were made in 1996 March with the MPIfR 1.3-mm 7-channel bolometer array. The bolometers are separated by 22 in an hexagonal arrangement surrounding the central pixel. The individual beamsize is about 11 (FWHM), and the typical sensitivity of 60 mJy s . The ON-OFF technique uses the wobbler with a beamthrow of 33, integrating for 10 s per subscan. The opacity was monitored every 1-1.5 hr with a skydip procedure, and was around 0.4 during the first night and 0.2 during the second. In both cases, the atmosphere was quite stable. The flux calibration was achieved by observing Uranus, and the uncertainties are again of order 20%. The fluxes of the six outer channels were averaged and subtracted from the central channel.

2.3. JCMT observations

The radio galaxies 1243+036 and MG 1019+0535 were also observed at the JCMT (the 15-m James Clerk Maxwell Telescope, Mauna Kea, Hawaii) using the single-element ³ He-cooled UKT14 bolometer (Duncan et al. 1990), coupled with a broad-band filter ( GHz; GHz FWHM). The observations were made using a 65-mm focal plane aperture, resulting in a FWHM beamwidth of 16.5. Sky emission was subtracted by chopping the secondary mirror in azimuth, at a frequency of 7.8 Hz, with a throw of 60.

The radio galaxy 1243+036 was observed on 1996 April 26 during excellent weather conditions. Calibration was taken from Mars (which set near the beginning of the shift) and the secondary calibrators NGC 2071IR, CRL 618, IRC +10216 (CW Leo) and 16293-2422. 3C 273 was used as a pointing source and bootstrap calibrator (observed eight times). The 384-GHz zenith optical depth was typically around 0.29, but it appeared to decrease towards the end of the shift (0.23). In total, 880 pairs of 16 s (i.e. 8 s in each beam) were obtained. The raw, uncalibrated dataset gave a S/N of 0.99. After despiking, calibration and statistical testing, the final result was mJy. The calibrated 800- m flux of 3C 273 was Jy; IRC +10216 (which is variable) was Jy.

The radio galaxy MG 1019+0535 was observed over a period of 5.5 hr during 1996 April 24. A total of 3 hr was spent on-source, the remaining 2.5 hr being devoted to calibration. Flux calibration was provided by regular measurements of IRC +10216, as was a determination of the 375-GHz zenith opacity. Observations of local pointing sources kept rms pointing errors below 3 and associated flux losses below 10%; the overall uncertainty in the flux measurement was therefore dominated by the poor S/N. After careful editing, the MG 1019+0535 measurements indicated a 375-GHz flux density of mJy - a marginal detection.

2.4. VLA observations

MG 1019+0535 was also observed at the Very Large Array (VLA), New Mexico. Radio continuum observations in the X (8.3 GHz), U (15.0 GHz) and K (23.0 GHz) bands were carried out in the D configuration on 1996 July 19 (with a bandwidth of 50 MHz), and in the Q band using the A configuration on 1996 December 26.

The total on-source time was 10 minutes each in the X and U bands, and 20 m in the K band. The visibility averaging time was 10 s in all bands. The phase was calibrated with calibrator source 1024-008 which was observed every 10 m, and 3C 286 was observed as a flux calibrator. The data were reduced using standard AIPS procedures. The used synthesized beam sizes, are listed in Table 2. Maps with a cell size of 0.5 were produced for each band. Maps were made and CLEAN ed using the MX routine within AIPS. The radio source was unresolved - no evidence was found for any extended emission. The total flux density, as measured from the maps in each band, is also given in Table 2. The major source of uncertainty for the measured flux density is the uncertainty in the flux scale, which we estimate to be of order 10%.

Table 2. VLA observations of MG 1019+0535.

The Q band observation was carried out over a period of 1.7 hr using twelve VLA antennas. The total bandwidth for the observations was 100 MHz, centred at 43.34 GHz (6.9 mm). Again, the observing and calibration procedures were standard. After checking the pointing accuracy of the antennas, brief observations of MG 1019+0535 were sandwiched and interspersed with measurements of 1055+018, a bright, compact calibrator. The flux density of the galaxy was tied to that of the calibrator (4.62 Jy) which, in turn, was tied to the flux density of 3C 286 (1.49 Jy on the shortest baselines). The noise level at the position of the galaxy agreed well with that measured using the calibrated visibilities, giving a upper limit of 1.94 mJy.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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