The Coalsack Loop

The most exciting feature of the new H survey is its potential to discover unique and unusual objects. In this section we present a newly discovered object which may be one of several new large scale objects detected by this survey.

The Coalsack Loop (Fig. 1) is a large ring of H emitting nebulosity surrounding the Coalsack Nebula, about 10 in diameter. It was first noticed on ESO/SERC Sky Survey plates, though it is only partly visible and very faint.

During February 1997, this region was imaged using the 16" telescope at Siding Spring Observatory. The telescope has been greatly modified for the Mount Stromlo CCD survey described elsewhere in these proceedings. Images covering 7 of sky are obtained using a 400mm f/4.5 lens in front of a 2Kx2K CCD, which have replaced the original mirror and tubing.

The image contained here is a composite of four fields, each being a 15 minute exposure through a 15Å H filter and using standard ESO/SERC Sky Survey field centers. Each image has been flat field corrected, and had bias and dark frames subtracted using standard IRAF procedures. On these images the loop is visible over about three-quarters of a circle. [SII] images have shown this emission to be weak.

This object is most likely the expanding remains of an old supernova remnant or HII region, and its location suggests that it may be interacting with the Coalsack. Further observations will be necessary to examine these possibilities, most importantly determining the object's distance. The Coalsack is composed of two clouds located at distances of 188 pc and 243 pc (Seidensticker & Schmidt-Kaler 1989). Adopting an angular size of 10 gives a diameter of 33 pc or 43 pc respectively if the Coalsack Loop is interacting with either cloud.

The Coalsack Loop has been identified as G303.5+0 in the radio continuum survey of Duncan et al. (1995). Here a near complete shell structure is visible. It is also clearly visible in images from the PMN 4850 MHz survey (Griffith & Wright 1993). These surveys show the presence of several large shell structures along the Galactic plane. The MOST survey will allow detailed examination of these objects' structure, as well as establishing if they are non-thermal. This H survey will reveal deeper and finer detail in the Coalsack Loop, as well as possibly detecting emission from the other observed radio structures.

© Copyright Astronomical Society of Australia 1997