Astron. Astrophys. 351, 1087-1102 (1999)

4. Kinematics

4.1. Large scale kinematics

Three position-velocity maps integrated over the ranges of Y indicated in Fig. 11a give a fairly complete summary of the kinematics of the molecular gas in M 31. The X-v map at Y 5´ (Fig. 11b) corresponds mostly to the upper part of S4, the region at Y -3´20" to the lower part of S4 (Fig. 11d), and the Y range between these limits to the tangent points along the major axis of S3, S4 and S5 (Fig. 11c).

Fig. 11. a1 and a2 are velocity-integrated maps of CO and H I respectively; the Y ranges over which the corresponding X-v maps below (b-d) were integrated are marked by the horizontal dashed lines. The first contour and the contour interval are 0.05 K-arcmin for the CO plots and 50 K-arcmin for the H I . Our fit to arm S4 (Sect. 5) is shown as dashed curves in b1 and d1 , while arrows in the same panels identify the nearly linear features corresponding to arms S3 and S5. The tangent points of S3 and S4 are indicated by the arrows on panel c1 .

The X-v plot for Y 5´ (Fig. 11b) reveals S4 as a continuous shallow loop corresponding to a pitch angle of 12^o (see Sect. 5). In addition to S4 however, emission is also detected from the arm S3, and possibly from S5. S3 appears as a nearly linear continuous structure below S4 between X = -30´ and X = 0´. As in the map, close to the minor axis the emission from S3 is difficult to separate from that from S4, owing to the steep inclination of M 31. Above S4, between X = -30´ and X = -60´, a third nearly linear component is seen. This emission comes from the small clumps located above S4 in the map, and may correspond to the outer arm S5.

Because the middle range, -3´20" Y +5´, corresponds roughly to the major axis, the X-v diagram there approximates rather closely the rotation curve of the galaxy, and is therefore fairly flat. Although the part of the diagram at X -30´ corresponds to S3, that at X -45´ to S4 and that at X -50´ to S5, the structure defined is continuous, and it is difficult to distinguish the various arms. We only note a small jump around -40´ which corresponds to the ill-defined bridge between S3 and S4 and to the location of the large OB association NGC 206. It is plausible that the perturbed kinematics of the molecular gas in this region, as well as the perturbed spatial structure (Fig. 8), result from the effects of the vigorous associated massive star formation. This diagram reveals that the molecular gas close to the center follows a perturbed kinematics as well: the gas at X = -5´ - -10´ is at a velocity of -500 , while the systemic velocity of the galaxy is at -315 . We discuss the kinematics of the central region in more detail below.

The X-v diagram at Y -3´20" also shows two components associated with S4 and S5. All the features in S5 are located below S4 in the map, as expected. It is worth noting that the intense CO clump at (-18´; -4´), though clearly aligned with S3 in the spatial map (Fig. 5), is aligned with S4 in velocity.

The kinematics of the molecular gas in the main disk follow very closely that of the atomic gas; no significant, systematic velocity offset is observed between the two. Like the integrated intensity maps, the position-velocity diagrams are smoother and more continuous for the H I than for the CO. Finally, we note that most of the well-defined complexes seen in the CO integrated intensity map are also seen as well-defined features in the X-v diagrams (see e.g. the complexes at (-46´, +7´), (-23´, +10´) or (-20´, -8´)). Though bigger than individual GMC's, these are well-defined structures both in space and velocity and they usually have H I counterparts.

4.2. The kinematics of the central region

The kinematics near the galactic center is best studied from a v-Y diagram integrated over the range in X where Fig. 11c revealed anomalous velocities, i.e. between X = -10´50" and X = -2´30" (Fig. 12). As this diagram shows very clearly, the molecular gas located in the inner 10´ (2 kpc) of M 31 follows a perturbed kinematics. Such anomalous velocities had been reported from optical spectroscopy by Pellet (1976), H I data by Whitehurst & Roberts (1972) and Brinks & Burton (1984) and CO data by Loinard et al. (1995). This gas is almost certainly not associated with the warped outer disk seen in projection through the main disk, but is rather gas located close to the center. Indeed, gas associated with the warped outer disk seen through the main disk at the position of the center ought to be at the systemic velocity of the galaxy, i.e. at a velocity of -315 . Stark & Binney (1994) proposed that the perturbed kinematics of the central region could result from the presence at the center of M 31 of a weak stellar bar, causing the gas to follow non-circular motions. Such models might explain not only the kinematics, but also the presence at the center of a "mini-spiral" and the distortion of the inner isophotes (e.g. Hodge 1992). Such a perturbed kinematics of the molecular gas is also detected at the center of the Milky Way (e.g. Dame et al. 1987) and has been interpreted as a consequence of a stellar bar (Binney et al. 1991). We note however that recent near-infrared ISO images of the bulge of M 31 at 5-µm reveal no trace of a bar (Lequeux et al. 1999, in prep.).

Fig. 12. Y-v maps of CO (grayscale) and H I (contours) integrated over a small range of X (-10.8´ to -2.5´) near the minor axis of M 31. The lowest CO grayscale level, and the spacings between levels is 0.005 K-arcmin. The H I contours are logarithmically spaced at 2.5, 6.3, 15.8, 39.8, 100 and 251 K-arcmin.

© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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