Asymmetry in the Vertical Distribution of Giant Molecular Clouds in the Carina Arm

Summary

The vertical distribution of molecular complexes along the Carina-Sagittarius arm has been studied on the basis of the giant molecular cloud (GMC) data compiled by Myers et al. (1986). The analysis indicates that the CO complexes are preferentially located below the formal Galactic plane. A separation of the sample into two groups: (a) GMCs associated with HII regions, and (b) GMCs without associated HII regions, establishes that group (a) shows, in average, larger z departure and mass than group (b). This result seems to suggest that the star formation activity in this major arm displays a vertical asymmetry which opens up interesting questions about the triggering mechanisms of star formation in spiral arms.

The density and location of young stars in major spiral arms and the relation to their parent molecular clouds are important to the understanding of how molecular clouds evolve and form stars in our Galaxy. In previous work (Alfaro et al. 1992, 1993) we analyzed the vertical structure of young open clusters (YOCs) along the optical segment of the Carina-Sagittarius arm, and its connection with the density of YOCs as representative of star formation activity. The main conclusions of that work can be summarized as follows:

1. A clear correlation between YOC density and z-departure from the formal Galactic plane is found when this density distribution is compared with the vertical structure. The cores of both supercomplexes are closely coincident with the two minima of the vertical profile, and the regions of lowest star-forming tracers appear associated with the relative maximum of z.