Review Questions

1. OVI absorption has primarily been observed in four redshift regimes, (i) the low-redshift Universe, z ≤ 0.5, (ii) the intermediate redshift desert, 1 < z < 2, (iii) Cosmic Noon, 2 < z < 3, and (iv) the pre-Cosmic Noon era,  3 < z < 3.5.  Each redshift range has its unique challenges, both due to technology limitations and due to astrophysics. For each redshift range, provide a brief summary of the challenges for surveying OVI-selected absorbers. Can OVI-absorbers ever be confidently surveyed for z > 3.5? Explain what challenges are faced at this higher redshift?

2. At Cosmic Noon (2 ≤ z ≤ 3), the properties (component column densities, Doppler b parameters, velocity clustering) of CIV and OVI absorption line profiles have been compared in some detail. Summarize the similarities and differences in the line profile properties of these two absorber populations during Cosmic Noon. What is inferred astrophysically from the relative behavior of CIV and OVI absorber components?

3. Consider Figure 11.13, which indicates that measurements of the OVI redshift path density, dN/dz, and mass density, ΩOVI, have significant scatter from survey to survey and that this scatter is much larger than the quoted uncertainties in the individual measurements. Discuss some possible reasons why the systematic errors currently far exceed the quoted statistical uncertainties.

4. Consider Figures 11.14(a), 11.15(a,b). and 11.16(b), which show that (i) the mean OVI column density does not evolve with redshift but that (ii) both the mean and the spread in the Doppler b parameter increases with decreasing redshift, while (iii) the ratio of the OVI to HI column densities is constant across the full range of cosmic overdensity (astrophysical environments). How are these facts interpreted, in other words, what is/are the astrophysical interpretation(s) of these observations?

5. In terms of the cosmic abundance of nitrogen relative to oxygen, what are the creation origins of "primary nitrogen" and "secondary nitrogen"?  What astrophysically can we learn by measure the [N/O] and [N/alpha] ratios as a function of redshift (where alpha means Type II SNe products like Mg, Si, and C)?

6. Compare and contrast NV absorption (i) in the Galactic plane in the Milky Way, (ii) in the CGM of external galaxies, (iii) in the low-redshift WHIM, (iv) in DLA-selected absorbers, and (v) in CIV-selected absorbers.

7. In the cooling flow model, how is the NV column density predicted to change with the cooling flow velocity? How might cooling flows help explain why NV absorption is rare in the photoionized phase of the CGM of galaxies?

8. What is the approximate range of  the gas phases for which the Ne+7  ion (NeVIII) has ionization fractions greater than or equal to 10% for (i) photoionization, and (ii) collisional ionization (see Figure 11.23)? What are some reasons that NeVIII absorption holds great promise for constraining absorber ionization models/conditions when OVI is also observed/present?

9. What are the observational challenges for observing NeVIII profiles and for obtaining accurate column densities, Doppler b parameters, and velocity components?

10. Briefly describe the general consensus of the physical nature of NeVIII absorbers based on theoretical investigations/simulations. Does theory suggest the majority of NeVIII absorbers are consistent with being collisionally ionized of photoionized?

Problems

1. Under construction