Neutral hydrogen near the galactic poles

Abstract

Surveys of 21-cm emission over the north and south galactic polar caps have been made with the Harvard 60-ft antenna. The northern polar survey described here includes only hydrogen near the local standard of rest velocity, observed with high-velocity resolution. The spectra have similar over-all characteristics over the whole region. When one analyzes them into Gaussian components, one finds several properties of interest. The velocity of the components has an exponential distribution of about 2 km/sec half-width about an aver- age velocity of zero. The average dispersion of the components is 2.5 km/sec, and the minimum observed dispersion is 1.1 km/sec. Several clouds have been found which have angular extent greater than the antenna beamwidth. If these clouds are assumed to be at a distance of 100 pc, they have linear diameters similar to other interstellar clouds but smaller densities and masses. The southern polar survey made with lower fre- quency resolution is compared with a similar survey made earlier in the northern polar cap. The amount of hydrogen is about the same in the two regions, and in each there is a preponderance of negative velocity hydrogen. In the south, the gas does not appear in two separate streams, one near zero velocity and the other between -20 and -50 km/sec, as in the north. There are in the south, however, several components with higher velocities, up to - 90 km/sec. These are compared to high-velocity clouds found by astronomers in the Netherlands.