Review Questions

1. Two "direct methods" for matching cloud models to observed column densities are discussed: (1) the chi-square approach, and (2) the density-metallicity locus approach.  Describe each from constraining observed quantities, to model building and preparation, to estimating the best model of the data.

2. Using the chi-square and/or the density-metallicity locus methods, what would be an indication that might compel you to infer that one of the elemental species has an abundance variation in the absorber being studied?  How would you estimate the magnitude of the abundance variation based on the cloud model predictions of the column densities?

3. When assessing a possible abundance variation, why is it important (two reasons) to know if the inferred variation is consistent or inconsistent with astrophysical expectations? In terms of Mg, O, Si, S, (the so-called ⍺-group elements), what are the astrophysical expectations of their abundances as a function of [Fe/H].  What is the astrophysics underlying these abundance patterns?  What other abundance pattern variations/patterns are known to have astrophysical origins? 

4. What are some immediate advantages of using likelihood functions and MCMC/Bayesian methods as compared to the direct methods? 

5. In your own words, describe how an MCMC algorithm employs "walkers" that yield posterior probability distributions of the model parameters. What are priors and what is the role of priors?  What is meant by "flat priors" verses, for example, "Gaussian priors"?

6. Describe some of the very first indications that multi-component multiphase ionization modeling was going to be required to accurately explain absorption line observations of both low and high ions.

7. In Section 30.3.3, we discussed multi-component multiphase Voigt profile fitting and illustrated an example in Figure 30.5.  For multiphase ionization modeling, such fits can be used as "Voigt profile-kinematic templates".  Briefly explain how this can be implemented, including why the low-ionization phase model would be "locked in" first and then the high ionization phase model would be determined. Regarding the HI column densities, what is the major challenge?

8. Briefly describe the special challenges with respect to OVI absorption profiles in the context of chemical-ionization modeling.  What are some insights that have been gained about some of the systematics that can occur by omitting or incorrectly treating OVI in the models?

9. Explain the evidence for poor mixing of metals from early multi-component multiphase modeling. Why does this further demonstrate to us that multi-component multiphase modeling is required if we are to obtain accurate insights into absorption line data?

10. Discuss some of the reasons as to why we may wish to entirely dispense with Voigt profile-kinematic templates. In other words, why might we be better off if we eliminate the step of conducting Voigt-profile decomposition prior to undertaking chemical-ionization modeling? What approach(es) might be pursued in which Voigt profiles and chemical-ionization modeling are combined into a single self-consistent method to interpret absorption line profiles?  

11. In your own words, provide a high-level summary of the approach taken by profile-based multiphase Bayesian chemical-ionization modeling (you might consult Figure 36.5 for a portion of your response). 

12. Consider Figure 36.6, which is a solution yielding a four-cloud multi-component multiphase model to a typical absorption system. Consider the posterior distributions. Examining clouds #1 and #2, we see they have identical line-of sight velocities and combine to create a single component/profile at v = 0 km/s. (i) Compare and contrast the four posterior distributions of these two clouds. (ii) Which scenario do you think is occurring here: (a) a single parcel of gas is multiphase and is dominated by two cloud conditions (after all, the posterior distributions exhibit substantial overlap), or (b) these are two unassociated spatially separated gas clouds that, per chance, have the same line-of-sight velocity in this galaxy halo. Provide some of your thoughts on which cloud properties best support your favored scenario and why. Balance these with your thoughts on which properties are less promising in support of your favored scenario and why.

Problems

1. Under construction