The Car–Sgr arm as outlined by superclouds and the grand design of the Galaxy

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Abstract
It is found that H I superclouds are regularly spaced along the very long Car–Sgr arm, the mutual distances being mostly 0.1 and 0.2 in units of the Solar distance to the centre. Such regularity is instrinsic to the grand design galaxies with density-wave spiral arms. A higher density of old stars and star clusters within the Car-Sgr arm is observed, implying a strong density wave. A symmetric second arm should exist in a grand design galaxy yet this arm is well seen only in the II quadrant, being behind the Per arm. The positions of most superclouds, the giant H II regions and GMCs over the Galaxy are compatible with a four-arm spiral structure with two less pronounced additional arms midway between the two principal arms; the nearby Per arm is one of the secondary arms. The pitch angle of such a grand design spiral pattern is 10°–12°. If this pattern does exist, the long segments of the arms have none the above-mentioned spiral-arm tracers. At any rate the Car-Sgr arm is certainly much brighter and more regular than all the others. The most probable arm class of the Galaxy is 9–12; it is multiarm or grand design galaxy.

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