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Astron. Astrophys. 325, 124-134 (1997)

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2. Observations and data reduction

The observations presented here were all made with the 30 m telescope of the Institut de Radio Astronomie Millimétrique (IRAM), located on Pico Veleta (Spain).

2.1. Molecular line observations

The observations of the [FORMULA] and the [FORMULA] lines were done in May 1995 and July 1996. We used one 3 mm (two during the second observing period) and two 1 mm SIS receivers available at the 30 m telescope simultaneously. The receivers were tuned for image sideband rejections [FORMULA] ([FORMULA] at 115 GHz). The system temperatures were 300 - 400 K at 115 GHz and 500 - 700 K at 230 GHz (in the [FORMULA] scale). In the following we use main-beam line brightness temperatures [FORMULA]. These are converted from the antenna temperatures, corrected for atmospheric attenuation and rear spillover, [FORMULA], through [FORMULA]. The beam efficiencies [FORMULA] are 0.73 and 0.45 for 115 and 230 GHz, respectively. The beamwidth was measured on Mars to be [FORMULA] for the [FORMULA] line and [FORMULA] for the [FORMULA] line. The backends used consisted of two [FORMULA] MHz channel filter banks, connected to one 3 mm and one 1 mm receiver, and an autocorrelator unit, connected to the other 1 mm receiver (and the other 3 mm receiver in July 1996).

The observations were centered on the major axis of the galaxy. Adopting the central position from the bolometer observations (given in Table 1), we observed several points out to a projected radius of [FORMULA] with [FORMULA] spacings near the centre and [FORMULA] spacings further out. Additionally we observed a few points above and below the major axis at distances of [FORMULA] and [FORMULA] from the plane. The observations were made by wobbling the subreflector at a rate of 0.5 Hz between the source and a reference position located between [FORMULA] and [FORMULA] in azimuth (depending on the observing position and the orientation of the source). Some scans at larger radii were observed in the position-switching mode with on- and off-position located symmetrically around the center. Cold load calibrations were made every 4 - 8 minutes.

During the CO observations we checked the pointing accuracy in two different ways. Firstly, we made pointing scans towards 1641+399 and 1418+546 every 1 - 2 hours. Secondly, we measured (every [FORMULA] 2 hours) small cuts perpendicular to the major axis at the center, consisting of three points at [FORMULA], and checked their symmetry, since the central point is expected to be strongest and the intensity of both off-axis points to be roughly equal. From these cuts and the pointing corrections made after each pointing scan we conclude that the mean pointing uncertainty is [FORMULA].

The data reduction was done in a standard manner using the GILDAS software package.

2.2. Bolometer observations

The [FORMULA] 1.2 mm observations were carried out in March 1995 with the 19-channel bolometer array developed at the Max-Planck-Institut für Radioastronomie, Bonn. The 19 channels are located in the centre and on the sides of two concentric regular hexagons, with a spacing between two adjacent channels (beams) of [FORMULA]. The central frequency and bandwidth of the bolometer are estimated to be 245 GHz and 70 GHz respectively (Guélin et al. 1995). For calibration purposes we have observed maps of Mars and Uranus during the bolometer observations. These maps yielded a conversion factor from observed counts to mJy/beam area of 0.32 mJy (beam area)-1 count-1. The beamwidth at this frequency is [FORMULA].

The continuum maps of NGC 5907 were observed in the Az-El coordinate system, with a scanning speed of [FORMULA] /s in Azimuth with data-acquisition every [FORMULA], and a subscan separation of [FORMULA] in elevation. During the observations, the subreflector was wobbled at 2 Hz in azimuth, with a beam throw of [FORMULA]. The starting point of each subscan was shifted a few arcseconds in azimuth with respect to the preceding one, which leads to a skewed shape of each single coverage in the Az-El space, with two edges of the maps parallel to the major axis of the galaxy. This as well as the use of different map sizes (between [FORMULA] and [FORMULA]) was done in order to ensure that each subscan covers the galaxy and at least [FORMULA] of blank sky on either side. We observed a total of fifteen single maps of NGC 5907, five centered on the optical centre (Barnaby & Thronson 1992), the others shifted [FORMULA] along the major axis to the northwest and southeast, respectively. Since the optical centre and the centre of the [FORMULA] emission (as found by our observations) differ by a few arcseconds, all offsets throughout this paper are relative to the latter position which is given in Table 1.

During the bolometer observing session the pointing accuracy was checked every 1 - 2 hours on 1418+546. The pointing corrections were always smaller than [FORMULA]. The atmosphere was relatively stable and the sky opacity was [FORMULA] most of the time (always smaller than 0.3). NGC 5907 was observed at relatively high elevations ([FORMULA] - [FORMULA]) what reduces possible calibration errors, which are typically of the order 15 %.

The data reduction was done with the MOPS software. A second order baseline was fitted to each individual scan in azimuth direction. The final restoration was done applying the "mask-and-shift" restoring method, as outlined in the "Pocket Cookbook" (Zylka 1996).

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998