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

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 = 13.7", = 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 ( 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 , except for the 492 GHz CI spectrum which was binned to 18 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, = /; relevant values for 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.

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 , vertical scale is in K. To convert to , multiply 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: is marked by solid lines, multiplied by three is marked by dashed lines.

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 = 530-1130 km s-1. Contours are in steps of 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.
Notes:
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 , the resulting r.m.s. noise and baseline deviations were of the order of 20 mK over most of the band.

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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