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

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6. Conclusions and perspectives

This paper presents a CO survey of a major fraction of the Andromeda galaxy (M 31) made at a resolution of better than 1´, and able to resolve individual spiral arms and molecular complexes. The CO emission is mostly confined to the two prominent, nearly continuous spiral arms S3 and S4 (Baade 1963), while additional emission features are found along the inner arm S2 and the outer arm S5. The arms S3, S4 and S5 are well defined in space and in velocity: they can be identified in position-velocity diagrams as fairly continuous shallow loops. The molecular gas closer to the center on the other hand obeys a perturbed kinematics, possibly due to the presence of a weak stellar bar.

Although M 31 is overall several times fainter in CO than the Milky Way, beyond a galactocentric radius of about 8-9 kpc (i.e. the solar radius), the Milky Way and M 31 are apparently very similar in their molecular content. The prominent molecular arm S4 detected here in M 31 is, in particular, similar in most respects to that portion of the Carina arm in the Milky Way lying at the same radius. The main difference in CO luminosity of the two galaxies is in the inner 7 kpc, where M 31 is very faint and the Milky Way quite bright. However, in spite of this difference, the ratio between the CO and the H I intensity seems to depend on the galactocentric radius in much the same way in M 31 and the Milky Way, rising steadily towards the center, at least down to R = 2 kpc.

At the intersection of the prominent arm S4 and the major axis, an offset between the gaseous arm and the arm defined by the UV light and the H II regions is observed: the OB associations are located several arcminutes downstream of the gaseous arm, while most of the H II regions are along the edge of the gaseous arm that faces the OB stars. Such an offset, also observed in the Milky Way and several other nearby galaxies, has been attributed to the passage of a spiral density wave. However, the streaming motions across the arm predicted by the density wave theory are not observed in M 31.

The data presented in this paper together with those we are currently gathering on the northern half of M 31 will be used to study in detail the structure and kinematics of the interstellar medium in M 31 and their relations to star-formation. The large scale kinematics of the ISM in M 31 for instance can much more easily be studied with CO than with H I , which usually does not disentangle gas in the inner and the warped outer disk. We will also be able to investigate on both small and large scales whether there exists a gas surface density threshold above which star formation sets in (Kennicutt 1998). Most such studies so far attempted included only H I data; in M 31, we will eventually have all the ingredients necessary for a complete study. New ISO mid-IR images and spectra (Cesarsky et al. 1998, Pagani et al. 1999, Lequeux et al. 1999, in prep) will greatly aid this investigation.

Molecular clouds in galaxies exhibit spatial and kinematic structure on a wide range of scales, and a description of the molecular component of a galaxy therefore requires observations with high angular resolution, high sensitivity, and wide angular coverage. The present survey of M 31, like that of the outer Milky Way by Heyer et al. (1998) with the same instrument, demonstrates the power of mm-wave focal plane array receivers to satisfy these conflicting requirements. We are currently mapping the other half of M 31 in CO with the new SEQUOIA array receiver on the FCRAO telescope; this new array is about 3 times more sensitive than QUARRY and will be upgraded from 16 to 32 elements within a year.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999