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Subsections

2 Observational material

2.1 The HI survey

We have analyzed a region of $6\hbox{$^\circ$ }\times 6\hbox{$^\circ$ }$ around NGC2359 using the 21 cm- HI survey of Hartmann & Burton (1997). This survey was made using the 25m radio telescope at Dwingeloo. The angular and velocity resolution of this dataset are 36$^\prime$ and 1.03 km s-1, respectively. The survey has been done with a sampling of 30$^\prime$ in galactic coordinates and have a sensitivity in brigthness temperature of 70 mK.

2.2 CO observations

The observations of the CO J $1 \rightarrow 0$ (115.271 GHz) and J $2 \rightarrow 1$ (230.538 GHz) lines and the J $1 \rightarrow 0$ (110.201 GHz) line of 13CO towards NGC2359 and its surroundings were made during 2000 February, using the NRAO[*] 12m radio telescope at Kitt Peak. The half power beam width of the telescope was 54 $^{\prime\prime}$ and 27 $^{\prime\prime}$ at the rest frequency of the J $1 \rightarrow 0$ and the J $2 \rightarrow 1$ lines, respectively.

The data were acquired simultaneously with two independent SIS receiver channels with orthogonal polarizations. The receiver temperatures were 250-300K and 500-600K for the J $1 \rightarrow 0$ and the J $2 \rightarrow 1$ lines, respectively. We used filterbanks (with 256 channels) and the hybrid correlator MAC (in the 2 IF mode, in parallel, 3072 channels) as backends. In the J $1 \rightarrow 0$ line, the filterbanks provided 100 and 250 kHz of resolution, corresponding to velocity resolutions of 0.26 and 0.65 km s-1, respectively. For the same line, the MAC correlator was setup with a total usable bandwidth of 300 MHz and with channels of 195.3 kHz of resolution (0.51 km s-1). In the J $2 \rightarrow 1$ line, the filterbank with channel widths of 250 kHz (0.33 km s-1) and 500 kHz (0.65 km s-1) were used. We also used for this line the MAC correlator tuned with an usable bandwidth of 600 MHz and frequency resolution of 390.6 kHz (0.51 km s-1).

 \begin{figure}
\par\resizebox{!}{125mm}{\includegraphics{ms10382f1r.eps}}
\end{figure} Figure 1: Distribution of the 21 cm- HI column density in a large region around NGC2359, obtained from the survey of Dwingeloo (Hartmann & Burton 1997). Every map has been constructed integrating over 2.1 km s-1 (2 channels), being the central velocity of integration indicated at the top right corner. Contour levels are 3 to 18 times 1019 cm-2. The small circle near the centre indicates the position and size of NGC2359. The Dwingeloo beam is also plotted in the bottom left map

Mapping of the J $1 \rightarrow 0$ and J $2 \rightarrow 1$ lines was made using the "on-the-fly'' technique. We have obtained three individual maps in the J $1 \rightarrow 0$ line and six in the $2 \rightarrow 1$ line. These individual maps were later reduced with AIPS and combined weighting them by their inverse square of rms. For our investigation, we finally used the MAC data for the J $1 \rightarrow 0$ line and the 500 kHz-filterbank data for the J $2 \rightarrow 1$ line. Typical final rms per grid point was 120 mK and 80 mK in the J = $1 \rightarrow 0$ and J $2 \rightarrow 1$ lines, respectively. We have also observed the J $1 \rightarrow 0$ line of 13CO and C18O at seven individual positions, with 7-10 min of integration time achieving a rms noise of 30 mK.

The pointing was checked at the begining of each observing session with five-point cross maps. The final data are pointed within 4'' of uncertainty. The data were calibrated using the chopper wheel method. Final intensity scale is in units of $T_{\rm R}^*$, e.g., corrected by atmospheric attenuation and instrumental losses (Kutner & Ulich 1981). The calibration was also checked off-line by comparing individual spectra of standard sources.


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