The Galactic Plane Survey Pilot Project:
The Interstellar Medium Environment of the W3/W4/W5 Star Formation Complex

Magdalen Normandeau

A multi-frequency Galactic plane survey was proposed as a means to unravel some of the mysteries of the interstellar medium. The main component of this survey is a complete mapping of atomic hydrogen emission from a sizeable fraction of the Galactic plane, as well as radio continuum emission at 408 MHz and 1420 MHz, using the Dominion Radio Astrophysical Observatory's synthesis telescope. In the observatory's standard mode of operation at the time the project was proposed, these observations would have required far too much time, over six years to cover a 70° x 8° area. It was therefore imperative that a new observing method be devised and tested to ascertain the feasibility of the survey as well as its potential usefulness.

In 1993, an 8° x 6° area of the sky covering the W3/W4/W5/HB3 region was imaged. These observations served to develop an efficient observing strategy for the Galactic plane survey, optimizing scheduling, calibration, sky-coverage, etc. The processing of the resulting large database stimulated the development of new software. The resulting images proved that the full survey would be of great scientific value for decades to come.

The astrophysical emphasis of this, the pilot project, deals with the effect of massive stars on the interstellar medium. The HII regions W3, W4 and W5, as well as the supernova remnant HB 3 are studied. The spectral line data cube reveals a wealth of shells, arcs, bubbles and filaments. Some of these can be linked to the HII regions or to HB 3. The origin of many of these structures can be traced back to the stellar winds of energetic stars. The most exciting discovery is that of a Galactic chimney above the W4 HII region. It is thought to have been blown by the stellar winds emanating from the stars of the open cluster OCl 352. This is the first clear example of a chimney in our galaxy. Other structures of note include two HI shells with, at the centre of each, a B-type post-main sequence star. It is thought that the stellar winds from these stars created these bubbles.