Astron. & Astrophys. 14, 1—262 (1971)

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

Are the Galactic Loops Supernova Remnants?

Elly M. Berkhuijsen, C. G. T. Haslam and C. J. Salter Leiden Observatory and N. R. A. L. Jodrell Bank

Received March 2, 1971

Observational evidence is presented on the nature of the Galactic continuum loops. Their small circle geo­metry is discussed. A relationship between neutral hydrogen features at intermediate and high latitudes and the continuum loops has been found. A link between Loop I and the runaway star f Ophiucus is suggested. The available evidence would seem to favour the supernova remnant hypothesis for the origin of the Galactic loops. Key words: continuum spurs — low velocity neutral hydrogen — North Polar Spur — runaway star — super­nova remnants

1. Introduction

The existence of ridges of enhanced continuum emission going to high latitudes has been known since the earliest days of radio astronomy. It has been shown (Quigley and Haslam, 1965), that to good approximation the three most prominent of these features follow arcs of small circles whose centres lie at intermediate latitudes, with the majori­ty of smaller ridges lying within these circles. Pre­viously the larger two of these arcs, or spurs as they are usually known, have been referred to as the North Polar Spur and the Cetus Arc. In this paper a terminology will be adopted whereby the term “spur” will refer to a single ridge feature projecting from the Galactic plane, whilst the name “loop” will be given to a combination of features which form a small circle. The name North Polar Spur will only be used for the intense continuum ridge which runs to high positive latitudes around l = 30°. The complete arc feature proposed by Large et al. (1966) will be called Loop I. The Cetus Arc will always be called Loop II, whilst the third object is already known as Loop III.

Over the years several theories have been suggest­ed to explain the nature of the spurs. Nowadays some of these can be rejected as being inconsistent with the improved observational data. However, none of those remaining are entirely convincing. Those currently gaining most support are:

1. The loops are remnants of:

1. Type I or Type II supernova outbursts (Han- bury Brown et al, 1960).

2. “Super” — supernova outbursts (Bingham, 1967).

2. The spurs join across the Galactic plane to form one helical structure (Rougoor, 1966).

3. The spurs are the “radio tracers” of the helical local Galactic magnetic field constructed on the basis of optical polarization measurements (Mathewson, 1968).

4. Bingham (1967) has suggested that the spurs may be bubbles or loops in the local magnetic field projecting from one side of the Galactic plane, and due to the instability of the field to cosmic ray pressure.

2. The Geometry of the Galactic Loops

One possible way of differentiating between the various theories of the nature of the Galactic loops is by a study of their geometries at low Galactic latitudes. The theories mentioned in Section 1 differ in their predictions of the paths these loops ought to follow across the Galactic plane. The hypotheses of Rougoor and Mathewson predict that the different spurs should join across the plane to form a helical structure. In contrast, if the spurs are caused by the instability of the interstellar magnetic field under cosmic ray pressure, loops of magnetic field should project from one side of the plane, and no continua­tion across the Galactic equator will be expected. If the spurs are supernova remnants they would be expected to appear from the plane stiff following the same small circles as in the other hemisphere, unless decelerated by increased density of material in the Galactic plane.

Yates (1968) has claimed to have followed Loop I along its small circle path to southern latitudes. His observations were made with a resolution of 3?5, and

a

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Fig. 1 a. Constant latitude 240 MHz scans across the North Polar Spur at one degree intervals between b — +10° and b — +18° (Haslam et al., 1964)

Fig. lb. Constant latitude 408 MHz scans across the North Polar Spur at half degree intervals between b = +4° and b = +10°

s the observed feature does not appear to follow the expected small circle too closely at the most southerly latitudes, it would be of great interest to obtain high resolution observations of the area.

Holden (1969) concluded from her 178 MHz ob­servations that there was no evidence for the exten­sion of the North Polar Spur into the Galactic plane below b = +11°. Constant latitude scans have been made at 408 MHz using the 250 foot telescope at Jodrell Bank. The observing system was identical with that used for the 408 MHz anticentre survey of Haslam et al. (1970). The scans were made in the longitude range 5° ^ l 40°, and were spaced at half degree intervals for — 2° ^ b ^ +10°. Figures 1 (a) and 1 (b) show constant latitude 240 MHz scans from Haslam et al. (1964), and 408 MHz scans for + 10° b ^ +4°.

Easily distinguished by its sharp outer edge at l ^ 30°, the main ridge of the North Polar Spur is seen to extend down to at least b = + 8°. This result is confirmed by the 820 MHz survey of Berkhuijsen (1971), which shows it continuing down to b = +6°. Below these latitudes the Spur becomes confused by Galactic plane radiation. The intense inner ridge, shown in Fig. 1 (a) as beginning at l & 22°, b « +14°, evidently continues to further than b = + 4° with a brightness at least equal to that of the main ridge. It is thus seen that this spur continues below b = +11° along the small circle path. The map of Berkhuijsen shows no connection between the North Polar Spur and the l = 50° end of Loop II.