2. Neutral Hydrogen and the Galactic Loops and Spurs

A brief report of this work has recently been pub­lished (Berkhuijsen et al, 1970). It will now be discussed in greater detail.

1. Loop I

The only previously noted relationship between the Galactic spurs and neutral hydrogen was by Lozinskaya (1964), who noted a deficiency of Hi on the ridge top of the North Polar Spur.

The work of McGee et al. (1963) gives contour maps of the peak brightness temperature of profiles of the low velocity neutral hydrogen. If the positions of the steep outer gradients of the continuum radia­tion from the North Polar Spur obtained from Cas­well et al. (1967), Williams et al. (1966), and Haslam et al. (1964), are plotted on these Hi maps (Fig. 4), these gradients are seen to coincide with a spur fea­ture in the neutral hydrogen distribution. Also plotted on this figure is the outer gradient of the continuum ridge at 8 ^ +10°, taken from Caswell et al. (1967). Ridges P and U from Large et al. (1966), are included up to the limit of that survey at a = 16 h. All these features are seen to have close coincidence with similar Hi features. The strong Hi background enhancement underlying the inner ridges could well be due to Gould’s Belt hydrogen (Davies, 1960). The Hi spur at l ^ 35° is also clearly apparent on McGee et al.’s contours of the velocities of the peak tempera­ture components, where it appears as a region of -f6km/s within a region of predominantly zero velocity.

The closely sampled survey of Grahl, Hachenberg, and Mebold (1968) shows the Hi distribution along b = + 30° within a velocity range of ± 60 km/s. Four features, narrow in longitude with high velocity dispersion, appear on this survey (Fig. 5). Each lies within five degrees of one of those longitudes at which Loop I and Loop III cross this latitude. It can be seen that the velocity dispersions of these features are greater than 40 km/s with longitude widths of less than 5°. The components near Loop I extend to posi­tive velocities, and those near Loop II to predom­inantly negative velocities. The high latitude obser­

vations of Tolbert and Fejes (1966), although never sampled at intervals of less than five degrees in l or b, show that all these features are probably extended in latitude parallel to the edges of the loops.

Recent work by van Kuilenburg (private com­munication) has confirmed the above findings in the region around l ^ 30°. He has made declination scans spaced every two degrees in right ascension using the 21-cm line receiver of the Dwingeloo tele­scope with a velocity resolution of 10 km/s. His results are given as a contour map of Hi surface density in the range ± 100 km/s with respect to the local standard of rest (Fig. 6). This confirms the association shown in Fig. 4 between the Hi spur and the continuum gradients. Examination of smaller velocity ranges shows that at low latitudes most of the gas has positive velocities of less than + 30 km/s. However, some patches occur between +30 and + 50 km/s, such as that a = 256°, d = +16°, which gives the + 40 km/s peak of Grahl et al. Components with velocities between — 10 and — 30 km/s are found particularly at higher latitudes, with concentrations appearing at a ^ 259°, 8 & +12°, at a ^ 249°, й^+17°, and also at aw 240°, 8 & + 25°, all on the outer continuum gradient. Clearly the velocity dis­persion of the Hi outside the continuum ridge of the North Polar Spur is high.

The region around the North Galactic Pole for b > + 80° has been surveyed for neutral hydrogen in the velocity interval ± 100 km/s by Dieter (1964). She found evidence for two components of the gas over the whole area, one moving with velocities near zero, and the other with velocities between — 20 and — 55 km/s. Figure 3 from her paper shows enhance­ment of Hi in the region + 80° < b < + 86°, 260° < l < 20° for both components. This region shows zero or small positive velocities for the low velocity peaks, compared with the small negative velocities outside it. The high velocity interval shows compo­nents between — 20 and — 30 km/s inside the region, with velocities solely between —30 and —55 km/s outside. It thus seems that even at these high Galactic latitudes one is probably seeing neutral hydrogen connected with Loop I.