Review Questions

1. Who was the first astronomer to detect radio emission from the Galaxy?  What year was that? Who was the astronomer to predict that the 21-cm emission line should be commonly detectable in the Galactic ISM? What year was that?

2. What transition in the neutral hydrogen atom gives rise to the 21-cm emission?  What is the physics that induces the upper and lower states for this transition? What frequency is 21-cm line in the radio spectrum?

3. What is the cold neutral medium (CNM) and what are its general gas phase characteristics (density, temperature)? What is the warm neutral medium (CNM) and what are its general gas phase characteristics (density, temperature)?

4. What physical ratio does the proxy quantity known as "spin temperature" measure? 

5. List and qualitatively describe the three physical processes that govern the equilibrium spin temperature in an astrophysical gas.

6. Section 6.2 introduces 21-cm absorption studies. Provide a brief summary of the types of astrophysics that can be studied using the 21-cm absorption line in quasar spectra.

7. What is the covering factor, Cf , and how does one attempt to measure it in 21-cm? Why is it that the covering factor is important in 21-cm radio studies of intervening absorbers as compared to UV/optical studies of intervening absorbers? 

8. When measuring the HI column density from 21-cm absorption in optically thin gas, there is a degeneracy between the covering factor and the spin temperature. Describe this degeneracy in terms of the way in which the HI column density scales with both quantities.

9.  It has been recognized that 21-cm absorption lines can arise in multiphase gas (CNM+WNM).  How does one decouple the two phases in an observed 21-cm absorption line? Describe how not properly treating this can bias the derived spin temperature measurement.

10. Explain how, in principle, you would carry out an experiment to measure redshift drift.

11. What four astrophysically interesting quantities govern the line strengths and redshift path densities of the predicted 21-cm forest lines over the redshift range z = 7 to z = 30? In other words, what four astrophysical quantities will we quantitatively learn about once we have solid measurements of the 21-cm forest?

Problems

1. Under construction