Review Questions

1. Consider Equation 33.1.  This is a very fundamental relationship between the measured column density of an ion and several key physical quantities of the absorbing gas, none of which can be directly measured.  In your own words, define each of the four terms on the right hand side of this equation. On what additional physics does the ionization fraction depend?

2. What is the definition of "ionizing photons"? For the purposes of quasar absorption line studies, what are the three most common sources of ionizing radiation?  In your own words, describe how the number density of ionizing photons, n_gamma, is determined from the mean intensity of the radiation field.

3.  What is the cosmic UVB? Briefly, describe the evolution in the shape and amplitude (at 13.6 eV) of the cosmic UVB. Why do you think the HI and HeII ionization edges decrease with decreasing redshift?

4. Compare and contrast the spectral energy distributions of quasars (QSOs), normal spiral and elliptical galaxies, and the local vicinity around starburst galaxies.

5. Consider the three relationships given in Equation 33.7.  For each, explain in words the particle density conservation that is being expressed. 

6. For a given atom or ion, what is the difference between its abundance fraction and its mass fraction?  Write out the equation for the standard log solar abundance represented by the notation [X/H].  Why is this notation so useful?

7. Charge conservation is the key constraint for determining the number density of free electrons in an ionized gas. Briefly explain the principles leading to Equation 33.26 and explain what this equation is stating.  Equation 33.28, which accounts for the total density of the gas, provides the fully self-consistent expression. Explain why it is a transcendental equation.

8. In words, what does it mean for a system to be in equilibrium? Of the types of equilibrium described in Table 33.1, which is the most stringent and why? Why do we describe various physical conditions in terms of "kinetic temperature," "radiation temperature," or "equilibrium temperature"?

9. What is the name of the thermalized kinetic distribution function? Consider Figure 33.3,  In your own words, what principle is panel (a) illustrating and what is panel (b) illustrating?  Include the behavior of the most probable particle speed in your answer. Also include a description of the change in the asymmetry/symmetry of the distribution of speeds relative to the most probable speed.

10. What is the expression for the average kinetic energy per particle in a gas for which the particle field is kinetically thermalized?  How do we obtain the total kinetic energy of the particles?

11. In terms of particle number densities, what is the simple relationship between the partial pressure of individual particles in a gas and the total pressure in the gas (accounting for its ionization as well)?  When we express this relationship in terms of particle mass densities, why do we need to account for the "mean molecular weight" of the particles in the gas? In this case, how is the ionization condition in the gas taken into account?

12. Explain the "massless electron" approximation and its practical utility. How accurate is this assumption in a fully ionized gas?

13. What is the equation of state for a gas and what macro variables does it describe? Consider Equation 33.76. Why is this very simple relationship so powerful for building intuition about the physical behavior of the equation of state?

Problems

1. Under construction