Astron. Astrophys. 347, 634-639 (1999)

2. Observations

We have used the 10-m Heinrich Hertz Telescope (e.g. Baars & Martin 1996) on Mt. Graham, Arizona between 1st and 18th April 1998 during medium weather conditions. A second observing session was available between 3rd and 17th December 1998. Several periods of clear weather () were available with the MPIfR two-channel 345 GHz SIS receiver. As backends we used the 1 GHz bandwidth 1024-channel Acousto-Optical Spectrometers supplied by the MPIfR which gives us a spectral resolution of 0.417 km s^-1. Receiver temperatures were between 120 K and 220 K (DSB) depending on the observing frequency. The system noise temperatures ranged from 400 K and up to 2000 K depending on weather and source elevation. The use of a double sideband receiver complicated our calibration procedures. However, as will be discussed later, good agreement has been found with several other published CO(3-2) observations made with other telescopes, so that we feel confident to quote main beam temperatures with an accuracy of better than 20%.

We have observed our galaxies with integration times of typically two minutes on-source, two minutes off-source. In April 1998 larger telescope movements from a low elevation planet to a galaxy gave pointing errors up to peak to peak which could be overcome by mapping each galaxy. By using maps of points we could overlap the different coverages. The relative pointing errors within a map of a galaxy were better than peak to peak. In December 1998, after the replacement of an encoder cable, the pointing stability of the HHT was vastly improved. Moving the telescope from one planet to another, many degrees away in the sky, gave positional differences of peak to peak in good () weather conditions. These positional accuracies could be furthermore maintained over many hours, allowing us to give an absolute positional accuracy of less than . In medium weather conditions () the pointing would degenerate to some peak to peak, still good enough for 345 GHz observations.

The beamwidth for CO(3-2) varied between and (FWHP) depending on the observing frequency. To achieve full sampling with this beamwidth we have used a mapping grid. Since only a double sideband receiver was available we placed the CO line in the lower sideband, thus ensuring that we had no line in the upper band. We used the wobbling secondary reflector with a beam throw between and , depending on the galaxy size and its orientation on the sky during observations.

We have observed the galaxies M82 and IC342 regularly which we used for a check of calibration consistency and pointing stability. The normal calibration was achieved by the insertion of a warm and cold (liquid nitrogen) load in the beam waist (where the beam emerges from the last mirror reflection, in front of the SIS receiver). Furthermore we have observed Galactic line calibrators as well. In particular, we have observed the stars  Cyg and V Cyg and used the flux values given by Stanek et al. (1995) at 345 GHz. These observations have in particular confirmed the value of 0.5 for the main beam efficiency of the HHT which was used by the SMTO staff. We have also intercompared the data of Mauersberger et al. (1999) with our data for calibration. The agreement with this data set was very good. The line intensity values published by Bash et al. (1990) for the galaxy M51 made with the CSO telescope agree with our values to within 15%. In addition, we know that the HHT has a surface accuracy of (B. Peters, private communication) which means that we do not expect problems in the calibration due to the error beam. In particular, we do not expect to have problems with emission in the sidelobes which will allow us to state that the CO(3-2) is indeed quite extended.

Taking all these factors into account and wanting to be on the conservative side, we estimate the absolute calibration accuracy to be better than 20%, which is consistent with variations commonly observed at sub-mm wavelengths. The relative accuracy within our maps is much better. We give our results here in main beam temperatures, , which are connected to antenna temperatures by the same calibration scheme as used at the 30-m telescope, , with (see above) and (D. Muders, priv. comm.).

In this paper we present the maps for the three galaxies M51, NGC 278 and NGC 4631. The CO(3-2) spectra of M51 are shown in Fig. 1. The extent of the mapped region is some which is a considerable part of the well-known optical image of this galaxy. In addition, we have made a scan of along the minor axis of M51. Extended CO(3-2) gas has been found in M51 in all positions where until now CO(2-1) and CO(1-0) line emissions were shown to be present by Garcia-Burillo et al. (1993) and Nakai et al. (1994). The CO(3-2) spectra for NGC 278 are shown in Fig. 2. The spectra show line emission in most of the field. The galaxy NGC 278 has an optical extent of only so that the CO(3-2) emission is present across most of this galaxy. The CO(3-2) spectra of NGC 4631 are shown in Fig. 3. These spectra again are present in the regions where lower frequency observations (Sofue et al. 1989; Golla & Wielebinski 1994) showed CO(1-0) and CO(2-1) line emissions.

Fig. 1. The 12(3-2) spectra in M51. Central position ,

Fig. 2. The 12(3-2) spectra in NGC 278. Central position ,

Fig. 3. The 12CO(3-2) spectra in NGC 4631. Central position ,

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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