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Dust and Molecules in the Milky Way

optical CO X-ray IRAS HI

Date:6.6.1994 Time:5:36 UT Exposure:60m
Field of View:100o x 65o Emulsion:Scotchchrome 400 Filter:none
Instrument:f=21mm 1/4.0 Place:Cerro Tololo Observer:S. Kohle

© Copyright by the observers

The CO overlay was made using the CO survey of the Milky Way . (Copyright by Tom Dame et al., Harvard-Smithsonian Center for Astrophysics.) The X-ray image was observed during the ROSAT All-Sky Survey, the IRAS image is from the Infrared Astronomical Satellite. The HI map is from Dickey and Lockman. These image were obtained from SKYVIEW.

Under a dark sky in the southern hemisphere you can see the center of the Milky Way from the viewpoint of the earth located in a spiral arm of the galactic disk. The light of stars in the disk and the bulge of our spiral galaxy is partly hidden behind dark clouds of gas and dust. As far as we know today there is an almost constant gas/dust ratio in the Milky Way, the dust beeing produced partly in circumstellar envelopes and enriched during several cycles of stellar evolution. The Interstellar Medium has a clumpy structure and in the most dense parts the hydrogen atoms are shielded by the dust from the interstellar radiation field. Therefore Molecular Clouds are able to form. This explains the nice correlation of the integrated CO emission (false colors in the overlay) and the dust clouds in the optical image.
The X-ray image is a three color composit in which emission in different Rosat energy bands is represented in different colors (1/4 keV = red, 3/4 keV = green and 1.5 keV = blue). As you can easily see the dusty molecular clouds also absorb the high energy X-rays, so that these regions in the sky appear darkly red.
The IRAS image shows the thermal emission of warm dust at 100 microns. But most of the dust-mass is cold and radiates at longer wavelengths, so that the anticorrelation of the dust emission shown here and the optical dust absorption is not very impressive. The HI image shows the distribution of neutral hydrogen.

Astronomical Institutes of the University of Bonn