In the Jodrell Bank 408 mHz anticentre survey the region in which the l & 155° end of Loop п1

e

1971A&A....14..252B

Fig. 2. The predicted small circle path of Loop П1 appearing at southerly Galactic latitudes near l — 150° is shown super­posed on the 408 MHz contours of Haslam et al. (1970). The contours are marked in units of °K

nters the Galactic plane has been fully mapped. Using ridge positions north of the plane measured from Turtle and Baldwin (1962), Seeger et al. (1965), Large et al. (1962), and the above anticentre survey, a small circle has been fitted by a least squares procedure. The ridge positions used cover a 180° arc of the best small circle. The coordinates of the centre of this circle, together with the radius and root mean square differences between the ridge positions and the small circle are given in Table 1.

The predicted path along which Loop III would appear at southern latitudes is shown superposed on the 408 MHz contours in Fig. 2. It can be seen that

this best small circle lies along the top of a ridge having spur-like appearance, with some fifteen degrees length by five degrees width. The brightness temperature of this feature has fallen below 2 °K by a = 02 h 15 m. This spur is also clearly seen on the 820 MHz map of Berkhuijsen (1971), the un­published 735 MHz map of Hogg and Pauliny-Toth, and the 13 MHz map of Andrew (1969). It is the only significant spur appearing from the plane at southern Galactic latitudes within more than ± 30° of the longitude predicted for the position of Loop III. No comparable emission is apparent along the path predicted by Rougoor to join Loop III at l = 155° to

Table 1. The small circle parameters of the galactic loops

Object	l (centre)	b (centre)	Diameter	R. M. S. Deviation	Arc Length
Loop I	329° ± Г.5	+17?5 ± 3°	116° ±4°	0?9	155°
Loop II	100° ± 2°	—32?5 ± 3°	91° ±4°	1?1	150°
Loop HI	124° ± 2°	+15?5 ± 3°	65° ±3°	1?7	180°
Loop IV	315° ± 3°	+ 48?5 ± 1°	39?5 ± 2°	0?8	190°

1971A&A....14..252B

Fig. 3. The small circle geometry of the Galactic loops. The crosses mark measured ridge positions used to compute the

small circles

*+40’

Fig. 4. The line of the outer gradients of the continuum radiation from the North Polar Spur is shown drawn on the maps of McGee, Murray and Milton which give contours of peak brightness temperature of the low velocity neutral

hydrogen profiles. The contour units are in degrees Kelvin. strong outer gradients, weak outer gradient,

— • — ridge top. The circles indicate the position of high velocity dispersion features from Grahl et al. (1968)

1971A&A....14..252B

50°

40°

30° l

170°

160°

150°

l 140°

Fig. 5. The four narrow-in-longitude, high velocity dispersion features appearing on the closely sampled Hi survey at

6 = 4- 30° of Grahl et al. (1968)

the l = 160° end of Loop II. These observations thns appear to show that Loop III emerges from the plane along its predicted small circle path, a result which is compatible with a supernova origin for this spur. It may be noted that the l ^ 155° ends of Loop II and Loop III do not fit the pattern of optical polari­zation found by Mathewson (1968).

The least squares procedure was also used to compute the small circle parameters for the other loops, using the most recently available data (see Table 1 and Fig. 3). The path for Loop I proposed by Large et al. (1966) was used, and the positions of the

peak of the main ridge were measured from Caswell et al. (1967), Haslam et al. (1970) and Large et al. (1966). For the Loop II small circle, ridge positions from Large et al. (1962), and Turtle and Baldwin (1962) were taken. For the loop formed by ridges D and E of Large et al. (1966) within Loop I, positions were taken from that survey. This feature, hereafter named Loop IV, has great similarity in geometry to the other loops. However, its small circle does not meet the plane, its lowest latitude being b ^ + 28°. Abo, its brightest segments are parallel, rather than perpendicular, to the plane. If indeed Loop IV is of

t

1971A&A....14..252B

he same family as the other three loops, its characte­ristics fit only those predicted by the supernova hypo­thesis. Further observations to investigate the nature of Loop IV would obviously be of great value.