A Survey of High-Latitude Molecular Gas in the Northern Galactic Hemisphere
D. Hartmann, L. Magnani, P. Thaddeus;
ApJ, 1998, 492, 205
ABSTRACT: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}}=2.5x10^{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&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.
KEYWORDS: ism: abundances, ism: clouds, ism: molecules, surveys
CODE: hartmann98