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

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1. Introduction

NGC 7331 is an isolated spiral galaxy of type Sb(rs)I-II with prominent dust lanes close to its centre (Kormendy & Norman 1979; Sandage & Tammann 1987). Table 1 summarizes the relevant parameters of NGC 7331. A ringlike distribution of dust surrounding the bulge was suggested by Telesco et al. (1982). Such a distribution is also apparent in radio continuum maps (Cowan et al. 1994), but only vaguely in HI maps (Bosma 1978; Begeman 1987). A strong ring signature in CO emission was claimed by Young & Scoville (1982).


Table 1. NGC 7331 parameters.
a) RSA (Sandage & Tammann 1987)
b) Dressel & Condon (1976)
c) Begeman (1987); Cowan et al. (1994)
d) Corresponding to [FORMULA] = 820 [FORMULA] (Begeman 1987)
e) Tully (1988); corresponds to [FORMULA] = 75 [FORMULA]
f) HI-derived parameters from Begeman (1987)
g) Begeman (1987) rescaled to D = 14.3 Mpc

Several galaxies are thought to host a molecular ring structure (see e.g. Braine et al. 1993), such as the ring in our own Galaxy discovered by Scoville & Solomon (1975) and the one in M 31 (Stark 1979; Dame et al. 1993; Koper 1993). The latter serves to illustrate a dilemma commonly facing the interpretation of CO maps, especially those of highly inclined galaxies where rings are most easily discerned: the conspicuous molecular structure may in fact consist of spiral arm segments that only in projection suggest a ringlike structure. Two-dimensional CO mapping of NGC 7331 by von Linden et al. 1996) and Tosaki & Shioya (1997) support the latter interpretation. Although molecular rings have been identified in or claimed for other galaxies, the case of NGC 7331 is of interest because it resembles M 31 in being a large spiral galaxy of relatively early type, containing a prominent stellar bulge. It has also been claimed to have, as M 31, very little CO emission inside its molecular ring (Young & Scoville 1982; Tosaki & Shioya, 1997). NGC 7331 even resembles M 31 in its high inclination (75o and 77o respectively - Arp & Kormendy 1972, Sandage & Tammann 1987). Its radio structure is a stronger version of that of M 31 (Cowan et al. 1994). NGC 7331 also contains a clear, but patchy radio continuum ring. Inside the ring, little or no radio emission is found, except for a compact nuclear source. The luminosity of this source is 3-4 times that of Sgr A, and a thousand times stronger than the nucleus of M 31. It is associated with a nuclear X-ray source (Stockdale et al. 1998). Ringlike distributions of interstellar dust are also seen at mid-infrared (Smith 1998) and submillimeter (Bianchi et al. 1998) wavelengths, but they are not nearly as evident at the far-infrared wavelengths inbetween (Smith & Harvey 1996; Alton et al. 1998).

An unusual characteristic of NGC 7331 is the rather low J=2-1/J=1-0 CO transitional ratio of 0.5-0.7 reported by Braine et al. (1993) and von Linden et al. (1996). This is quite different from most other galaxies observed in CO, where the two transitions are usually of similar strength. However, such low transitional ratios have also been found for individual dark clouds in the central parts of M 31 (Allen & Lequeux 1993). These were interpreted as evidence for very cold ([FORMULA] 5 K) and tenuous ([FORMULA] 100 cm-3) molecular clouds by Loinard et al. (1995), but Israel et al. (1998) showed that they are more likely caused by filamentary gas at temperatures [FORMULA] K present at both low and high densities. Given the similarities between NGC 7331 and M 31, it is of interest to investigate whether such a state of affairs also applies to the central region of NGC 7331.

In this paper, we present a fully sampled map of NGC 7331 in the J=2-1 12CO transition over an area of 1.3´ by 3.5´. In addition, we have measured the first three 12CO and 13CO transitions as well as the 492 GHz CI transition towards the central region of the galaxy, allowing us to narrow down the permitted range of the apparently unusual physical conditions in the center.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999