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L. Magnani (U. Georgia), L. E. Smith (UCLA), D. Hartmann, P. Thaddeus (Harvard-Smithsonian CfA)
We surveyed the Southern Galactic Hemisphere at b \le -30\deg\ in the CO(1--0) emission line to determine the filling factor of molecular gas at high Galactic latitudes and to search for heretofore unknown molecular clouds. The southern Galactic Hemisphere was sampled on a locally-Cartesian grid with 1\deg\ (true-angle) spacing in Galactic longitude and latitude. Of the 11,478 points in the sampling grid, the 4982 positions which rise at an elevation \ge 30\deg\ from Cambridge, MA, the site of the 1.2 m millimeter-wave telescope which was used for the survey, were observed to an rms of Tmb \le 0.2 K. One-hundred-forty-five distinct CO(1--0) emission lines were observed along 135 lines of sight. Of these detections, 71 are new and 74 are associated with 21 previously-cataloged southern hemisphere high-latitude clouds situated within the survey boundaries. The completeness of the survey was estimated to be ~70% based on the detection rate of previously known clouds and Monte Carlo simulations. The surface filling factor corrected for the incompleteness of the sampling grid is 0.041, a factor of 10 greater (depending on the cloud-size distribution) than that found in the Northern Galactic Hemisphere at b \ge 30\circ. Adopting as the CO to H2 conversion ratio N{\rm H}_2/W\rm CO = 2.5 \times 1020 cm-2 [K km s]-1, the mass surface density of molecular gas in the southern hemisphere at high Galactic latitude is 0.2 M\sun\ pc-2. The scale height of the 145 detections is ~150 pc, greater than the value for local dark and giant molecular clouds. Thus, the high-latitude molecular clouds may be the local analogue of the thick molecular disk observed in the inner Galaxy.
The author(s) of this abstract have provided an email address for comments about the abstract: loris@zeus.physast.uga.edu