When we look at the various images of spirals (Fig. 17.3), we see that the spiral arms are traced by strings of bright HII regions. This suggests that the spiral arms are sites of enhanced massive star formation. We would like to apply the ideas we developed when studying star formation in our galaxy to help us understand other spiral galaxies. In turn, our understanding of other galaxies will help our analysis of our galaxy. We can ask a
21 cm map of the Andromeda Galaxy, M3I, made using the Westerbork interferometer in the Netherlands. [Elias Brinks, Sterre wacht, Leiden University]
number of questions about star formation in spri-ral galaxies.
(1) What is the large-scale distribution of star forming material? How does it vary in the disk with distance from the center? How does it vary with distance from the central plane of the disk?
(2) Is the interstellar medium concentrated into the spiral arms?
(3) How do the sizes of molecular clouds compare with those in our galaxy? Are the physical conditions within the star forming clouds the same?
In studying the interstellar medium of our galaxy, or any other, radio observations play an important role. Except for the nearest galaxies, single-dish radio observations do not provide much spatial detail. However, with the extensive use of interferometers we have now obtained very detailed maps of many galaxies. Continuum observations can be used to study the positions of HII regions and young supernova remnants, both signs of relatively recent star formation. Studies of spiral structure have been limited by poor angular resolution for single-dish studies. However, sufficient resolution is available to study nearby galaxies.
M31 is the nearest spiral, so it provides the best opportunity for studying the interstellar medium in detail. At 700 kpc distance, 1 arc sec corresponds to a linear size of 3 pc, so a 100 pc long giant molecular cloud would subtend an angle of 30 arc sec. This corresponds to the resolution of typical single-dish millimeter telescopes. To study the large-scale distribution of molecular material, we could use single-dish observations, but to study individual clouds we have to use millimeter arrays. To study the HI, single-dish observations at 21 cm do not give sufficient resolution, so we must use arrays.
Fig. 17.12. shows an interferometer map of the large-scale distribution of HI in M31. The large-scale molecular distribution is shown in a single-dish CO map in Fig. 17.13. One of the problems in studying the spiral structure in M31 is that it is tilted so it is hard to trace accurately the spiral arms as they would appear if we were looking from overhead. The large-scale distribution of star forming regions in M31 is shown by the FIR image in Fig. 17.14.
The single-dish CO observations of spirals reveal a sharp falloff in brightness with radius, similar to that of the visible light. The falloff in CO emission may indicate the true gas distribution. However, it may be due to the fact that the gas cools, and therefore radiates less strongly where there are fewer stars to heat it. There may
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