A Survey of High-Latitude Molecular Gas in the Northern Galactic Hemisphere

We surveyed the northern Galactic hemisphere (NGH) at b ≥ 30° 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° (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° in Cambridge, MA, the site of the 1.2 m millimeter-wave 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 deg2 suggest that the survey is ~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 ≤ -30°. Adopting as the CO to H2 conversion ratio NH2/WCO = 2.5 × 1020 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 ≥ 30° is found to be largely free of molecular gas.