Review Questions

1. What was Lyman Spitzer's hypothesis and what were his physical arguments?

2. Describe the first experiments (facilities, targets, people involved) that confirmed Spitzer's hypothesis and found OVI absorption (and therefore a low-density hot gas phase) in the Galactic ISM.

3. Describe the first experiment that confirmed OVI absorption was present in the high-redshift (z ≈ 2.7) Universe.  What technique was used to make this measurement and what role did CVI absorbers play? Explain why we detected OVI absorption at high redshifts well before we were able to detect it at low redshifts and in the extended halo of the Milky Way.

4. Describe the early history of the theoretical predictions of the WHIM. What is the definition (gas phases: density and especially temperature) of WHIM gas? 

5. WHIM gas can be detected in OVI, OVII, and OVIII absorption using FUSE, HST, and Chandra. Provide a short review of the first observations of these ions confirming the existence of the WHIM.

6. Consider Figure 10.3 (you may wish to examine the color version in Nelson et al. 2018).  According to these simulations the WHIM has a complex and segregated morphology. Briefly describe this morphology, include in what region(s) of the WHIM OVI, OVII, and/or OVIII absorption is expected to dominate?  Incorporate Figure 10.4 and also provide the mean gas phase (density temperature) where the majority of the mass of OVI, OVII, and OVIII bearing gas resides.

7. If you aim to estimate Ωb from ΩOVI, what conversion factors do you need to apply?  What are the key astrophysical considerations, assumptions, and uncertainties involved in applying these conversion factors?

8. What is the simple (single parameter) observational definition of a broad Ly⍺ absorber (BLA)? Why are BLAs believed to be good probes of  WHIM gas and what ionization conditions and gas phase(s) of the WHIM do they preferentially select? Describe some key observational findings with regard to OVI absorption in BLA absorbers.

9. What are fast radio bursts (FRBs)? Describe/explain how they are useful for obtaining measurements of Ωb that are more direct and certain than using ΩOVI measurements.  

10. Understanding the range of gas phases and dynamical structures in which CIV, SiIV, NV, and especially OVI absorption arise has been and remains challenging. Very briefly (at a high level), compare and contrast (i) cooling flows, (ii) static warm plasmas, (iii) shocked plasmas, (iv) conductive interfaces, and (v) turbulent interfaces as possible sites that could explain high-ionization absorption observations.

Problems

1. Under construction