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

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

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...492..205h&link_type=abstract

Astrophysical Journal v.492, p.205

Astronomy and Astrophysics

Astronomy

28

Ism: Abundances, Ism: Clouds, Ism: Molecules, Surveys

Scientific paper

We surveyed the northern Galactic hemisphere (NGH) at b >= 30 deg 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 deg (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 deg 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 deg. Adopting as the CO to H2 conversion ratio N_{{H}2}/W_{{CO}}=2.5x1020 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&sun; 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 deg is found to be largely free of molecular gas.

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