Astron. Astrophys. 341, 256-263 (1999)

2. Observations

2.1. Observations at the Heinrich-Hertz-Telescope

The observations of the ¹²CO 3-2 line were conducted with the 10-m Heinrich-Hertz-Telescope (for a description of the HHT, see Baars & Martin 1996) on Mt. Graham in southern Arizona during two observing sessions on December 18/19, 1997 and January 6/7 1998 under very good weather conditions. The spectrum of M 82 was measured in May 1998. The zenith optical depth at 345 GHz was in most cases 0.2; only in a few observations was it as high as 0.4. Most sources have been observed at actual optical depths ; only for Cen A was 1.1. The latter source was also partially blocked by trees, so the absolute calibration of Cen A is much less secure than for the other sources. The atmospheric fluctuations and the atmospheric model introduce a calibration uncertainty not greater than 5%.

A 2 channel SIS receiver was equipped with a 1024 channel acousto optical spectrometer with a total bandwidth of 1 GHz. The second channel, which has a more reliable calibration, was used for this study. System temperatures for a single sideband were typically 400-1500 K (). The beamwidth was 21". The receiver was sensitive to both sidebands. The CO line was observed in the lower sideband. Any imbalance of the gains in the lower and upper sideband will increase the calibration errors. Tuning the receiver with different parameters for the backshort resulted in variations of the intensity of IC 342 of up to .

All results are given on a main-beam brightness temperature () scale. This is related to the antenna temperature via ; it is the same calibration scheme as used at the IRAM 30-m telescope (Downes, 1989). The main-beam efficiency, , has been measured by the SMTO staff in November 1997 to be 0.50 () toward Saturn, which at that time had a size of 18". The surface of the HHT is within m (i.e. at 345 GHz) very accurate. There are no strong error beams at 345 GHz. This reduces the effect of source size on the appropriate source coupling efficiency. The forward hemisphere efficiency, , is close to 1. On that scale, a spectrum of the CO line taken toward IRC+10216 yields and . The uncertainty of introduces a 6% calibration uncertainty.

The spectra were taken using a wobbling secondary mirror with a beam throw of in azimuth. Scans obtained with reference positions on either side were coadded to ensure flat baselines. Baselines of order zero or one were subtracted from the data. The scale is determined by measurements of an ambient load and the sky, which are interspersed with the spectral line observations. The receiver temperature was determined from measurements of an ambient load and a cold load (at liquid nitrogen temperature) after each retuning of the receiver. The variations of the sky opacity were also monitored by a tilting radiometer at 230 GHz.

Our pointing was based on the pointing measurements obtained by the SMTO staff, which gave an RMS deviation of measured pointing sources from a best model of . For a 21" beam, the resulting calibration uncertainty could result in an underestimate of the intensity of a point source of , and less for extended sources. Although most of our observed positions should be exact within 5" we cannot exclude that a few sources have pointing errors as large as half a beamwidth. This would result in an underestimate of the calibration of 50%.

The calibration uncertainty resulting from the above factors (under the assumption of a 5" pointing error) is , which is consistent with variations commonly observed at sub-mm wavelengths.

2.2. Observations at the IRAM 30-m telescope

Some of our sources were observed at the IRAM 30-m telescope in the and 2-1 transitions of ¹²CO in May 1998. The wobbling secondary mirror was used with a beam throw of 4´ in azimuth. Both lines were observed simultaneously using two 512 MHz filterbank spectrometers. The beamwidths of the 30-m telescope at 115 and 230 GHz are 21 and 12". Except for M 82, only the 115 GHz data were used in this paper since the incomplete sampling of our maps did not allow to obtain 2-1 data convolved to a 21" beam.

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998
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