Radio tracers of spiral structure

We can view the situation on a larger scale by using radio observations to look at the distribution of interstellar gas. This allows us to see across the whole galaxy. We can utilize kinematic distances,

Star Formation Radio Astronomy

Fig 16.14.

Spiral structure of the galaxy as determined from giant molecular cloud complexes inside the solar circle. The sizes of the circles indicate the masses of the complexes, as indicated in the upper right.The 4 kpc and Scutum arms are drawn from the 21 cm maps.The Sagittarius arm is drawn as it would best fit the CO data. [Thomas Dame, CFA/Dame,T.M. et al., Astrophys.J., 305,892, 1986, Fig. 9]

Fig 16.14.

Spiral structure of the galaxy as determined from giant molecular cloud complexes inside the solar circle. The sizes of the circles indicate the masses of the complexes, as indicated in the upper right.The 4 kpc and Scutum arms are drawn from the 21 cm maps.The Sagittarius arm is drawn as it would best fit the CO data. [Thomas Dame, CFA/Dame,T.M. et al., Astrophys.J., 305,892, 1986, Fig. 9]

studying the spiral structure in inner parts of our own galaxy, especially with the distance ambiguity. Or, it may be telling us that the spiral pattern in the inner galaxy is not that well defined. We will see in Chapter 17 that, in many other spirals, the pattern gets stronger as one moves farther out.

Outside the Sun's orbit, the approach is more direct. Since we have a rotation curve for the outer part of the galaxy and there is no distance ambiguity, it is easier to trace out the large-scale structure. It is in the regions outside the Sun's orbit that we see the best evidence for spiral structure, with some features being traced over at least a quarter the circumference of the galaxy (Fig. 16.15). There is a growing confidence that the outer part of our galaxy is a four arm spiral. This work is still going on.

Much of our understanding of spiral structure comes from comparing our galaxy to other

Locations of Mapped Clouds but must still deal with the distance ambiguity. Initial radio studies of the interstellar gas and spiral structure involved the 21 cm line. Again, long connected features have been identified.

With the discovery of molecular clouds, it was hoped that they would reveal the spiral structure of our galaxy. This is because the optical tracers of spiral structure we see in other galaxies - OB associations, HII regions, dust lanes - are all associated with giant molecular clouds. A number of groups have carried out large-scale surveys of emission from CO throughout the galaxy. We look at some results in Fig. 16.14. Most of the work involved material inside the Sun's orbit. The problem is that the distance ambiguity makes it difficult to place uniquely all of the emitting regions. One approach has been to take specific models for spiral arms and predict the outcome of CO observations. Again, pieces of arms have been identified, and again we see pieces of arms. This may be telling us about the difficulties of

Fig 16.15.

Molecular clouds outside the Sun's orbit and spiral structure. (a) First and second galactic quadrants.The cloud masses are denoted by the symbols, shown at the lower left.The circle at 13 kpc is drawn in for reference.

Fig 16.15.

Molecular clouds outside the Sun's orbit and spiral structure. (a) First and second galactic quadrants.The cloud masses are denoted by the symbols, shown at the lower left.The circle at 13 kpc is drawn in for reference.

galaxies. Therefore, we will leave further explanation of spiral structure until Chapter 17, when we look at other galaxies.

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