Neutral hydrogen in the haloes of the Galaxy and the LMC

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Abstract
A new low-noise 21-cm line receiver installed on the Parkes 64-m telescope has led to the detection of the H I counterpart of ionized and neutral elements in the galactic halo. The observations show that very low-intensity H I exists in three regions at average LSR radial velocities of – 40, + 59 and + 131 km s−1 which, if the halo is corotating with the disc, can be interpreted as components of the galactic halo at considerable distances below the galactic plane. Two additional groups of low-intensity H I features at heliocentric velocities of + 197 and + 360 km s−1 possibly indicate a similar halo around the Large Magellanic Cloud. Comparison of the column densities obtained by the ultraviolet observations with the International Ultraviolet Explorer and the present H I observation show that O I in the halo is depleted by a factor between 2 and 10 compared to solar abundance. Of the elements which should be dominant in H I regions Al and Si are depleted by a factor of ≲ 2 and Fe is close to the solar abundance. Ground-based data on Ca II show that it is depleted by factors of 4 to 3000, with the greater values occurring at velocities associated with the galactic disc and the Magellanic Clouds. Thus Ca appears to follow the typical pattern of being depleted less in the halo than in the spiral arms.

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