On the comparison of spiral structure as delineated by gas and by stars

Extract

In an earlier article^* it was pointed out that the galactic radial motions ΔE (R, l) of the very young stars did not show the uniformity of motion to be expected from a smooth regular expansion of the Galaxy. Instead, the very young stars were found to show large-scale regional peculiar motions; these regional peculiar motions are displayed in Figure 1. In addition to regional peculiar motions and the space distribution of stars, Figure 1 also shows the spiral structure delineated by neutral hydrogen gas. As is customary in such diagrams, the space distribution of gas and the space distribution of the stars are not in good agreement. As various investigators have mentioned, stars and gas appear to define different spiral arms. However, such a conclusion is not warranted by data such as those employed in construction of Figure 1. In Figure 1 (as is invariably the case in earlier published diagrams of the same sort) two distance scales have been employed in the construction of the diagram. The distances of the stars have been derived from photometric data; the distances of concentrations of neutral hydrogen gas have been derived from measured hydrogen gas radial velocities and a galactic rotation curve. It should therefore come as no surprise if there are disagreements between hydrogen spiral arms and star spiral arms. Any regional peculiar motion of a gas concentration directly becomes an error in the inferred distance of the gas concentration.