A survey of high-latitude molecular gas in the northern Galactic hemisphere

We surveyed the northern Galactic hemisphere (NGH) at b greater than or equal to 30 degrees in the CO (1-0) emission line to determine the surface filling factor of molecular gas at high Galactic latitudes and to search for heretofore unknown molecular clouds. The NGH was sampled on a locally Cartesian grid with 1 degrees (true-angle) spacing in Galactic longitude and latitude. Of the 11,478 points in our grid, we observed all 10,562 positions that rise to an elevation above 30 degrees in Cambridge, MA, the site of the 1.2 m millimeterwave telescope that was used for the survey. Only 26 lines of sight showed CO emission. Monte Carlo simulations based on our sampling grid and with cloud sizes, in a uniform distribution, ranging from 0 to 2 deg(2) suggest that the survey is similar to 70% complete. Power-law distributions yield fractional completenesses that are typically a factor of 2 lower. The surface filling factor, corrected for the incompleteness of our sampling grid, is 0.004-0.008, depending on which cloud size distribution is used. These values are substantially lower than what is found in the southern Galactic hemisphere at b less than or equal to -30 degrees. Adopting as the CO to H(2) conversion ratio N(H2)/W(CO) = 2.5 x 10(20) cm(-2) (K km s(-1))(-1), the mass surface density of molecular gas in the north ranges from 0.015 +/- 0.009 to 0.035 +/- 0.020 M. pc(-2). With the exception of four fairly significant aggregations of clouds (the complexes associated with the Polaris flare, Ursa Major, Draco, and L134), and a handful of isolated cloudlets, the northern Galactic hemisphere at b greater than or equal to 30 degrees is found to be largely free of molecular gas.