Review Questions

1. What defines a Lyman limit system (LLS) and, considering Figure 4.1 and Table 4.1, what are some additional characteristics of LLSs? Describe what the characteristic or defining absorption feature of a LLS looks like in a quasar spectrum. Why is Lyman limit absorption sometimes called a "Lyman-break"?

2. What defines a damped Ly⍺ absorber (DLA) and, considering Figure 4.1 and Table 4.1, what are some additional characteristics of DLAs? Describe what the characteristic absorption of a DLA looks likes in a quasar spectrum.

3. According to Figure 4.3, what is the approximate average HI column density for absorption arising in the intrafilamentary medium (IFM) at (i) z = 0, (ii) z=2, (iii) z = 4, and (iv) z = 6?

4. Explain the reasoning as to why, when it comes to measuring the metallicity in DLAs, we can safely ignore ionization corrections and still obtain a first-order approximation of the abundance [Fe/H] directly from the ratio of the column densities FeII/HI, where FeII is singly ionized iron and HI is neutral hydrogen. [Note: using iron as an example here, the logic applies to other elements as well].

5. Consider Figure 4.7 and the evolution of the Ly⍺ forest absorbers from z = 7 to z = 0.  (i) Qualitatively describe the evolution in the flux values from the Epoch of Reionization to Cosmic Noon and then from Cosmic Noon to the Current Epoch. HINT: you might consider the evolution of the transmitted flux PDF. (ii) Across these same cosmic epochs, qualitatively describe the ionization evolution and morphologies of both the neutral and ionized intergalactic medium.

6.  What change(s) in the behavior of the measured effective optical depth of HI with redshift led to the interpretation that the Epoch of Reionization ended around z = 5.3? Briefly explain the analysis that led to (or supported) that conclusion and what it implies about the difference between the ionizing background radiation above and below this redshift.

7. What is the astrophysical interpretation of what causes a Ly⍺ transmission spike?

8. Consider Figure 4.10. Qualitatively compare and contrast the quantitative behaviors of the redshift path densities dN/dX of the IGM/IFM, LLS, sub-DLAs, and DLAs.

9. Consider Figure 4.12. Qualitatively compare and contrast the z ≤ 1 metallicities [X/H] of sub-LLS, LLS, sub-DLAs, and DLAs. 

10. Consider Figure 4.13. Qualitatively compare and contrast the evolution in the metallicities [X/H] of LLS/sub-DLAs, QSO-DLAs, and GRB-DLAs. What behavior may be occurring for DLAs at z ≥ 4.5?

11. Why does the mass-density of HI, ΩHI, decrease with decreasing redshift? Or, equivalently, what does the fact that the mass-density of HI decreases with cosmic time tell us about the Universe?

Problems

1. Given that the average ΩHI is about 0.001, what fraction of Ωb is the average HI mass density of the universe?