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Astron. Astrophys. 351, 10-20 (1999)

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2. Observations

Details relevant to the observations are listed in Table 2; the system temperatures given are the means for the respective runs. Observations in the J=1-0 transition were obtained with the IRAM 30 m telescope in service mode, at the optical and radio centre positions respectively, separated by [FORMULA] = 13.7", [FORMULA] = 14.0". The 13CO observations were bracketed by the 12CO observations.


Table 2. Observations Log

All other observations were carried out with the 15m James Clerk Maxwell Telescope (JCMT) on Mauna Kea (Hawaii) 1. Up to 1993, we used a 2048 channel AOS backend covering a band of 500 MHz ([FORMULA] at 230 GHz). After that year, the DAS digital autocorrelator system was used in bands of 500 and 750 MHz. Resulting spectra were binned to resolutions of 4-10 [FORMULA], except for the 492 GHz CI spectrum which was binned to 18 [FORMULA] on order to obtain a sufficiently high signal-to-noise ratio. Only linear baseline corrections were applied to the spectra. All spectra were scaled to a main-beam brightness temperature, [FORMULA] = [FORMULA]/[FORMULA]; relevant values for [FORMULA] are given in Table 2. Spectra are shown in Fig. 1 and summarized in Table 3. Position A (indicated in Fig. 2) is that of the radio nucleus, the actual galaxy center. Along the minor axis, the beam just fills the space between the ring components. Positions B and C are close to the minor axis at deprojected radii R = 4.5 kpc and R = 8.2 kpc respectively; position B is centered on the CO ring. Because of the pronounced tilt of the galaxy, 21" circular observing beams sample elongated ellipses in the plane of the galaxy, covering a range of 5.55 kpc in the minor axis direction. The beams covering positions B and C overlap (see Fig. 2); positions A and B are essentially independent.

[FIGURE] Fig. 1. Spectra observed towards positions A, B and C in NGC 7331 (see Sect. 2). Top row: J=1-0 CO; second row: J=2-1 CO; third row: J=3-2 CO; bottom row: [CI]. Horizontal scale is LSR velocity in [FORMULA], vertical scale is [FORMULA] in K. To convert to [FORMULA], multiply [FORMULA] by 1.35 (J=1-0 CO), 1.45 (J=2-1 CO), 1.72 (J=3-2 CO) and 1.89 ([CI]), respectively. Note: [FORMULA] is marked by solid lines, [FORMULA] multiplied by three is marked by dashed lines.

[FIGURE] Fig. 2. Left: Digitized sky survey image of NGC 7331 with observed positions A, B and C (see Table 3) marked. Size of circle corresponds to 230 GHz beamsize. Right: Distribution of J=2-1 12CO emission in NGC 7331, integrated over a velocity range [FORMULA] = 530-1130 km s-1. Contours are in steps of [FORMULA]dV = 4 K km s-1. Observed positions A, B and C are indicated by circles. Table 1.


Table 3. CO and CI line intensities in NGC 7331.
a) From Young et al. (1995);
b) From Elfhag et al. (1996);
c) Extrapolated value;
d) From von Linden et al. (1996)

For the J=2-1 mapping observations, the integration time was typically 400 seconds per spectrum, on and off the source. After binning to a velocity resolution of 5 [FORMULA], the resulting r.m.s. noise and baseline deviations were of the order of 20 mK over most of the band.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999