EXPOSING THE STUFF BETWEEN STARS:

A Progress Report on the Canadian Galactic Plane Survey Mapping Our Milky Way Galaxy

Canadian Galactic Plane Survey (CGPS) maps, delineating the gas and dust between stars, reveal the complex ``ecosystem'' of our Galaxy, the Milky Way. Highlighting a strip of our Galaxy stretching across 2000 light-years, a recent panorama (Map 1) exposes, for example, the detailed structure of a giant ``spur'' known as Lynds Bright Nebula 679 (Map 2). In their presentation at the American Astronomical Society Meeting in Atlanta today, Steve Gibson and Russ Taylor of the University of Calgary focus on hydrogen gas clouds that are cold enough to appear as interstellar silhouettes (Map 3) in this map. This is the first time the cold hydrogen clouds have been seen in detail, emphasizing the superb quality of the data collected by National Research Council of Canada's Dominion Radio Astrophysical Observatory (DRAO). And while the origin of these clouds remains a mystery, they are suspected of being the first step in the formation of stars out of the stuff between stars.

Taylor says, ``Understanding our origins, ultimately from the gas and dust between the stars, requires an understanding of the Galactic ecosystem. Counterbalancing forces within the dusty, gaseous interstellar medium govern the cycle of star birth and death. Therefore this medium effects the evolution of the Galaxy as a whole. The CGPS, carried out by an international consortium of universities and observatories, is attempting to reveal the details of this complex ecosystem.''

The Synthesis Telescope (ST) at DRAO, an array of 7 radio antennas in Penticton, B.C., is responsible for detecting atomic hydrogen gas for the survey. Invisible to optical telescopes, this gas can only be revealed by observations of faint radio waves emitted at a wavelength of 21 centimetres. The distribution of antennas along a 600-metre rail track generates an angular resolution of 1 arcminute, which is more than a factor of 10 better than that in any previous study of the Milky Way. Although the survey is still not complete, the CGPS data started becoming public, at the Canadian Astronomy Data Centre, August 1999.

Map 1 displays the stuff between stars in the Perseus Spiral Arm of our Milky Way Galaxy by assigning different colours to radiation not detected by the human eye. Besides DRAO radio data (assigned combinations of blue, orange, yellow, and green), this composite image includes Infrared Astronomical Satellite (IRAS) data (coloured pale pink), and data (grey-blue) from the Westerbork Synthesis Radio Telescope (WRST) in the Netherlands.

The location on the night sky of this panoramic strip is displayed in Diagram 1 while its position in the Milky Way relative to our Sun is presented in the ``top view'' schematic in Diagram 2.

The diffuse nebulosity that fills the Map 1 panorama from left to right is hydrogen gas, which is distributed along the mid-plane of the Galactic disk and runs across the middle of the image. However CGPS astronomers used the DRAO array to isolate the gas in the Perseus Arm, 6500 light-years distant, from the rest of the gas in the disk. Overlaid on these hydrogen data are IRAS images of the dust that intersects our line of sight through the disk. The point-like sources (WRST) are mainly distant galaxies that have very energetic cores called Active Galactic Nuclei.

Far from a homogeneous environment, the interstellar medium in the Perseus Arm shows a highly disturbed and seemingly chaotic appearance. This region has been shaped partly by the ongoing formation of massive, hot stars near the hot dusty clouds (appearing pink) of the W3, W4 and W5 regions right of centre, and perhaps also by the passage of a large scale spiral shock wave as the matter in the Perseus Arm orbits around the centre of the Galaxy.

A giant "spur" seen in the Perseus Arm panorama, and detailed in Map 2, is interstellar gas and dust emerging from the disk of our Milky Way Galaxy. Seen against a backdrop of distant quasars, Lynds Bright Nebula 679 is over 200 light years in extent and apparently driven out of the disk of the galaxy by giant interstellar shock waves. Lying in the constellation Perseus, the spur is over 2 degrees in angular extent, about four moon diameters. At the surface of the spur lies an interface between the cool and hot matter forming the diffuse interstellar medium. On the left side, facing the plane of the galaxy the material in the spur consists of cold hydrogen gas and warm dust. On the side facing away from the plane of galaxy the spur is being evaporated by ultraviolet radiation. The hot gas being driven off the spur can be seen as a faint green halo arising from hot, fluorescing hydrogen gas.

Despite the violent motions in the interstellar medium and the impact of energetic radiation, pockets of quiescence exist where gas clouds can cool and condense, leading to the formation of the next generation of stars. Map 3 magnifies a section of Map 1 exposing clumps and filaments of cold hydrogen gas silhouetted against the hotter diffuse hydrogen. These interstellar silhouettes appear purple-ish due to the colours assigned to the DRAO hydrogen dataset. The origin of these clouds is still a mystery, but they may be first step in the evolution of the diffuse interstellar gas to a cold, dense state in which molecules can coalesce, clouds collapse, and new stars form.



MAP CREDIT:

Dr. Jayanne English (CGPS/STScI) using data acquired by the Canadian Galactic Plane Survey (NRC/NSERC) and produced with the support of Russ Taylor (U. of Calgary).

NOTES TO EDITORS: SCIENCE CONTACT INFORMATION:
Interstellar Silhouettes:
Dr. Steven J. Gibson
Department of Physics and Astronomy
University of Calgary
2500 University Drive, N.W.
Calgary, Alberta
Canada T2N 1N4
Phone: (403) 220-8015
Fax:      (403) 289-3331
Email:  gibson@ras.ucalgary.ca

Other:
Dr. A. Russ Taylor
Department of Physics and Astronomy
University of Calgary
2500 University Drive, N.W.
Calgary, Alberta
Canada T2N 1N4
Phone: (403) 220-5385
Fax:      (403) 289-3331
Email:  russ@ras.ucalgary.ca

IMAGE CONTACT INFORMATION:
Dr. Jayanne English
Space Telescope Science Institute
3700 San Martin Dr.
Baltimore, MD 21211
USA
Phone: (410) 338-4352
Fax:      (410) 338-5090
Email:  jenglish@stsci.edu

(Note that since Map 1 is 3204 x 1764 pixels, it is well-suited to 2 page spreads.)

INTERNET ACCESS:
On January 14, 2000 full resolution tiffs of these 3 images and 2 diagrams, along with other images from the CGPS will be available at
http://www.stsci.edu/~jenglish/cgps/pr/