Astron. Astrophys. 359, 1139-1146 (2000)

2. CO observations and data reduction

New CO observations of N 66 have been secured with the SIS receivers of the Swedish-ESO Submillimeter Telescope (SEST) at La Silla during November 1998. These receivers have approximate single-band system temperatures of 400 K and 300 K at 115 and 230 GHz respectively, in units. The image band rejections are respectively 20 and 15 dB. In order to convert the into main-beam temperatures , one has to divide by the main-beam efficiency of 0.70 and 0.60 at 115 and 230 GHz respectively. We performed simultaneous observations in the CO(1-0) and CO(2-1) emission lines over the ionized region. The map was done in the beam-switching observing mode with a 20"20" spacing. The velocity resolution was 0.13 km s^-1 and 0.056 km s^-1 per channel at 115 GHz and 230 GHz, respectively. The integration time was 4 min per map position. The rms noise achieved in a single channel was 0.1 K at both frequencies.

For these observations, we concentrated on sensitivity and due to the limited amount of observing time the region was sampled every 20" instead of 10" as in the previous observations. The map is thus undersampled given the HPBW of 23" of the SEST at 230 GHz. However the sensitivity permitted the detection of CO over most of N 66 where nothing was detected with the older receiver set-up. The Key Programme CO observations towards N 66 were done in CO(1-0) and they covered a 9´9´ area with a 60" grid spacing. CO was detected only in two positions and this emission was fully mapped in CO(1-0) and CO(2-1) as reported in Rubio et al. 1996.

Fig. 3 shows a contour map of the integrated CO(2-1) line. For building this map and the following ones, we have combined the old observations of the molecular cloud in the NE (Paper I) with the new observations. Fig. 4 displays the contours of the integrated CO(2-1) line superimposed over the map in the ISOCAM LW2 filter (5.0-8.0 µm). There is an excellent correlation between the CO and AIB emissions.

Fig. 3. Contour map of the integrated CO(2-1) line emission in the region of N 66. The sampling in CO is over a 10"10" grid in the upper left part of the figure, which is built from the old, low-sensitivity SEST observations while the rest of the figure is built from the new, high-sensitivity observations sampled every 20"20". The contours levels are from 0.67 to 3.0 by steps of 0.33, and from 5.0 to 20.0 by steps of 2.5, in units of K km s-1. The temperature scale is . Grid points show the area covered by our old and new observations.

Fig. 4. Contour map of the integrated CO(2-1) line emission in the region of N 66, superimposed over the map in the ISOCAM LW2 filter (5.0-8.0 µm). Contour levels as for Fig. 3. For designation of the peaks of mid-IR emission, see Fig. 8.

Fig. 5 displays channel maps of the CO(2-1) emission every 5 km s^-1, also superimposed over the LW2 filter image (5.0-8.0 µm), smoothed to the 20" CO resolution. These maps show that the strong emission to the NE is limited to the range 155-165 km s^-1, while the emission associated with N 66 extends from 140 to 165 km s^-1, with a complex velocity structure. The CO emission towards the HII region which is seen in the same velocity range as the NE cloud, is clearly spatially separated. Thus, there is no connection between these emissions, and there is no reason to believe that they are physically associated.

Fig. 5. Contour maps of the CO(2-1) line emission in the region of N 66, in velocity channels 5 km s-1 wide, superimposed over the map in the ISOCAM LW2 filter (5.0-8.0 µm), smoothed to a resolution of 20". The LSR velocity range is indicated on each map. For the channel maps, the contours are from 0.42 (3 ) to 1.67 by steps of 0.42, then from 2.5 to 8.33 by steps of 0.83, in units of K km s-1. The temperature scale is . For the map of total CO (140-165 km s-1) in the bottom right, see the caption of Fig. 3.

Table 1 presents the parameters of the CO(2-1) line in the direction of the well-defined, apparently unresolved peaks identified on Fig. 8. They were obtained by gaussian fitting of the line profile. Due to the incomplete sampling, the true intensity can be somewhat larger, and the data in this table are indicative only.

Table 1. Parameters of the CO(2-1) line in the direction of peaks identified in Fig. 8. The temperature scale is .

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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