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

3. Results

3.1. CO distribution

In the J=1-0 and J=2-1 transitions, the integrated intensities of both CO isotopes are rather similar for positions A and B, indicating a fairly smooth central distribution of relatively strong CO. This is consistent with the large (50) fraction of flux found to be missing by Tosaki & Shioya (1997) in their J=1-0 CO interferometer map. The results presented here and by von Linden et al. (1996) are, however, inconsistent with the folded major axis profile obtained by Young & Scoville (1982) and shown in more detail by Young et al. (1995), which exhibits both a pronounced lack of CO at the centre and a strong peak at a radial distance of 45" (3 kpc). From the data in Table 2 and the results obtained by von Linden et al. (1996), it appears that the central integrated value given by Young et al. (1995) is too low by more than a factor of two. Thus, there is no significant central `hole' in the distribution of CO emission , at least not on the scale of our 20" beam.

A full-resolution contour map of the J=2-1 CO intensity, integrated over the velocity range of 530 to 1130 is shown in Fig. 2. The map shows a good overall resemblance to the J=1-0 CO map obtained at slightly lower resolution by von Linden et al. (1996). This is also true for the major axis position-velocity diagram (not shown here). In Fig. 2, the elliptical outline of a low-contrast ring around the center can be discerned; peaks of CO emission occur at positions 30" north and 50" south. The latter two more or less correspond to the radial distance of the molecular ring proposed by Young & Scoville (1982). The map covers the brightest part of the optical image also whown in Fig. 2 (for better images, see panel 40 in the atlas by Sandage & Bedke 1988). In this image, a large, overexposed bulge is surrounded by dust lanes and irregular spiral arms traced by HII regions. The CO maxima at = -50" and +30" fall on either side of the bulge, and the ring traces dusty spiral arms close to the bulge, especially on the western side. Most of the reddening of NGC 7331 occurs in this western spiral arm (Telesco et al. 1982; Bianchi et al. 1998). On the eastern side of the CO map, faint emission due to a more distant major spiral arm is seen as well. The HI map obtained by Begeman (1987) at a very similar resolution shows an incomplete `ring' of neutral hydrogen. The CO emission from the outlying spiral arm coincides with a relatively bright part of this HI `ring'. Most of the CO emission is, however, well inside it and coincides with the radio continuum ring mapped by Cowan et al. (1994). The main CO peaks are at the northern and southern extremities of the radio continuum ring.

In the velocity-integrated single-dish CO map (Fig. 2), the ring is only weakly visible. Its presence is more clearly revealed in the interferometer map of Tosaki & Shioya (1997) and in Fig. 3. This figure shows the distribution of J=2-1 ¹²CO over the same region, but now integrated over velocity bins of 40 only. Between velocities = 600 and 1000 , the maps show a double structure. The double-peak structure seen in Fig. 3 extends over most of the part of NGC 7331 characterized by rigid rotation (cf. von Linden et al. 1996). A similar pattern, with the limitations imposed by interferometric techniques, is also evident in the channel maps published by Tosaki & Shioya (1997).

Fig. 3. J=2-1 12CO channel maps of NGC 7331. Emission is integrated over velocity bins of 40 ; central velocities are marked in the panels. Contours are in steps of dV = 2 K km s-1.

3.2. Line ratios

The ¹²CO/¹³CO isotopical ratios of 6-7 are somewhat low compared to typical values around 10 found in most other external galaxies, but given the low IRAS ratio of 0.3 (Rice et al. 1988) this is in line with the results obtained by Aalto et al. (1991). We find a ¹²CO J=2-1/J=1-0 ratio of 0.540.10 in the center, consistent with the results obtained by Braine et al. (1993) and von Linden et al. (1996). Both J=1-0/J=2-1 intensities and ratio decrease away from the center. The CO transitional ratios observed in NGC 7331 are rather different from those of most other galaxies, where the lower J transitions usually have similar velocity-integrated intensities (cf. Israel & van der Werf 1996). In contrast, CO intensities in the centre of NGC 7331 decrease rapidly with increasing J level (Table 3). The J=2-1/J=1-0 ratios suggest subthermal excitation either at relatively low excitation temperatures or at very low column densities. However, the observed J=3-2/J=2-1 ratios of 0.6 or higher indicate the presence of a certain amount of warm molecular gas. Very low temperatures are also unlikely because they imply ratios substantially closer to unity than is observed. We also note, in Table 4, a similarity between the ratios applicable to the D 478 cloud in the central parts of M 31 and to the emission from NGC 7331, notwithstanding the factor of 400 difference in beam surface area.

Table 4. Integrated line ratios in the centre of NGC 7331.
Notes:
a) Dark cloud in M 31; see Allen et al. (1995); Loinard & Allen (1998); Israel et al. (1998).

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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