At a distance of 690 kpc, the Andromeda galaxy 1 (M 31, NGC 224; Fig. 1; Table 1) provides a unique opportunity to study the distribution and kinematics of gas and dust in a spiral system, as well as the relation of gas and dust to spiral structure and star formation. Its proximity allows studies of its stellar and interstellar constituents with a linear resolution achievable in no other spiral, except M 33; its high inclination (about 13o from edge-on) makes it difficult to determine the distribution of clouds along the minor axis, but otherwise facilitates the study of kinematics. The distribution of young massive stars (Magnier et al. 1993) and H II regions (Pellet et al. 1978) are known throughout the disk, and complete maps of atomic gas (e.g. Brinks & Shane 1984), infrared dust emission (Xu & Helou 1996) and radio continuum emission (Walterbos et al. 1985; Bystedt et al. 1984) have been obtained at angular resolutions of 1´ or better.
Table 1. General properties of M 31
At the same resolution, the distribution of molecular gas in M 31 has been poorly determined until recently. Previous CO observations are listed in Table 2, and summarized in Fig. 2. Sparse observations along the major axis by Combes et al. (1977a, 1977b) and along the minor axis by Stark et al. (1979) and Sandqvist, Elfhag & Lindblad (1989) suggested that the CO intensity was highest in the direction of the dark dust lanes at 10 kpc from the galactic center. Two large regions have been mapped at an angular resolution of 100" using the AT&T 7-m telescope: an 18´ 15´ field near the southwest tangent of the Population I ring (Stark et al. 1981) and a 20´ 4´ field along an eastern spiral arm (Ryden & Stark 1986). In both instances, the emission was resolved into molecular spiral arms 1-2 kpc wide, with dust and atomic counterparts.
Table 2. Previous CO observations of M 31 ordered by date. S1 and S2 correspond to two peaks in the map by Stark et al. (1981) and R1 and R2 to two peaks in the map by Ryden & Stark (1986) - see Fig. 2.
Most subsequent observations were made at higher angular resolution but covered much smaller regions near emission peaks in the AT&T maps. These observations generally aimed at studying the properties of individual GMC's (e.g. Vogel et al. 1987, Wilson & Rudolph 1993) or the relation of atomic to molecular gas inside spiral arms (e.g. Lada et al. 1988). In addition, Allen & Lequeux (1993) and Loinard et al. (1996a) detected faint but broad CO emission profiles associated with dust clouds in the inner 5 kpc of M 31, which they interpreted as an indication of large amounts of very cold molecular gas. Based on a sensitive survey along the inner major axis made with the IRAM 30-m telescope, Loinard et al. (1995) suggested that as much as 60 of the inner disk of M 31 could be covered by such cold gas.
The only complete CO survey to date of M 31 was done with the 1.2-m telescope at CfA (Dame et al. 1993); its large beam () and sensitive receiver were well adapted to the mapping of such a large, faint galaxy. This survey demonstrated that the CO emission peaks in a broad Population I ring, and provided important information on the total molecular content and global molecular distribution in M 31. In particular, it showed that the average CO intensity of M 31 is fainter than that of the Milky Way by a factor of a few. However, the modest linear resolution of the CfA survey (1.7 kpc along the major axis) did not allow the distribution of molecular gas at the scale of spiral arms and individual molecular complexes to be determined. Until now, only a few percent of the entire galaxy has been observed at an angular resolution (better than 1´) that would allow the arms to be studied (Fig. 2). Besides the present survey, a large-scale "on-the-fly" survey at 21" resolution has recently been undertaken with the IRAM 30-m telescope (Neininger et al. 1998 - Fig. 2).
In this paper, we present a CO survey of most of the southwestern half of M 31 performed with the FCRAO 14-m telescope (HPBW = 45"), some of which was presented and analyzed by Loinard et al. (1996b) 2. In Sect. 2, we will describe the telescope, the observing techniques, and the reduction procedures used for the CO observations, as well as some additional data required for the analysis. The spatial distribution of CO intensity will be described and compared to that of other tracers of the interstellar medium and regions of star formation in Sect. 3. The kinematics of the molecular gas will be discussed and compared to that of the atomic gas in Sect. 4, while the information gathered on the spiral structure using the CO emission will be described in Sect. 5. Sect. 6 summarizes the main results of the survey and lists some perspectives.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999