11.4.2. Magellanic Clouds
As already mentioned, the observations of the Magellanic Clouds with the Parkes 210-foot dish have the highest physical resolution so far attained. The Magellanic Clouds cover some 800 square degrees of sky and, at a distance of 50 kpc, are the nearest extragalactic objects to our own galaxy. A low-resolution (2.°2 beamwidth) HI survey (Hindman, Kerr, and McGee, 1963) showed that the Large and Small Clouds (LMC and SMC) are embedded in the same HI envelope. The total HI mass of the system is some 1.5 × 109 M; 5.4 × 108 M is associated with the LMC and 4.8 × 108 M with the SMC.
The 210-foot survey of the LMC (McGee and Milton, 1966) revealed 52 HI complexes of mean HI mass 4 × 106 M and diameter 575 pc ( 1 HI atom/cc). The HI complexes are closely associated (in position and velocity) with HII regions and OB stars, but do not correlate with the stellar clusters.
Of 90 large HII regions cataloged by Henize (1956), 61 are closely associated with the HI complexes. The supergiant OB stars with measured velocities also have a high correlation in position and velocity with the HI complexes.
From an analysis of the rotation of the LMC and the position and velocity distribution of the population I objects, McGee and Milton (1966) proposed a spiral structure for the LMC. The total mass derived from rotational and random motions is greater than 6 × 109 M, giving the LMC a fractional hydrogen content of 5 to 9%. The 1410-MHz radio continuum also correlates well with the integrated hydrogen contours.
The neutral hydrogen distribution in the SMC is rather smooth, with three major concentrations reaching peak brightness temperatures of 150, 110, and 100°K, merging into a high-level background. This smooth distribution is in striking contrast to the clumpy distribution in the LMC. The most interesting feature of the 210-foot survey (Hindman, 1967) is the presence of what would appear to be three massive (1 to 2 × 107 M) expanding neutral hydrogen shells of diameter 1 to 2 kpc, which could be the result of supernova explosions. The SMC is also thought to be rotating, but is only slightly flattened. The estimated mass is 1.5 × 109 M, with some 30% neutral hydrogen. As in the LMC, there is a correlation between the distribution of neutral hydrogen and the bright stars. About half of the cataloged HII regions (Henize, 1956) are located in the vicinity of the three major gas concentrations.