Review Questions

1. For absorption line analysis, why do we normalize the continuum before we start searching for absorption lines and measuring equivalent widths and column densities?

2. Explain what the key advantage is if one employs orthonormal functions for continuum normalization in view of the error propagation applied to absorption line measurements.

3. Explain, qualitatively, the key steps and concerns when co-adding spectra? What is flux conservation and why is it critical? 

4. What is a "flux decrement"? Describe the uncertainty model for the value of the flux decrement in a given pixel? 

5. When objectively searching for statistically significant absorption features in spectra, two methods have been discussed: (1) the Aperture Method, and (2) the LSF Optimized Method. (i) Provide a brief qualitative explanation/description of how each is implemented.  (ii) Include the role of the significance-level threshold, N𝛔.

6.  For the Aperture Method, what property of the spectra guides our choice for the number of pixels in the "aperture"? What are some shortcomings of the Aperture Method?

7. For the LSF Optimized Method, an aperture is selected as well; what considerations guide our choice of  the number of pixels in this aperture? 

8. For both the Aperture and LSF Optimized Methods, why do we compute the aperture sums using the "equivalent width spectrum" and its uncertainty spectrum rather than, for example, using the pixel-by-pixel flux decrement spectrum? HINT: what very important information is directly provided by the "equivalent width uncertainty spectrum"?

9. Though the formal definition of the equivalent width is an integral of flux decrements, describe in practice how we can alternatively compute the line center, equivalent width, and uncertainty in the equivalent width using direct summations over pixels.

10. When implementing the autocorrelation spectrum, what choice are we making in order to convert observed wavelength in the spectrum to redshift? If an autocorrelation spectrum examining a doublet-selected absorber (like MgII 2796,2803, or CIV 1548,1550) yields a value of "5" in a redshift pixel, what does this mean?  

11. What are some options when choosing how to determine the systemic redshift, zsys (or equivalently, the absorber redshift, zabs ) of an absorption system? What rest-frame quantity does this choice fix for subsequent analysis of the absorber kinematics?

12. Consider Table 29.3.  The equivalent widths of some transitions in some velocity ranges are quoted as upper limits. How does one determine when measurements are upper limits?  How does one compute their values? What role does N𝛔 play?

13. What is an "unresolved" absorption line versus a "resolved" absorption line? Explain how we determine if an absorption line is unresolved or resolved.

14. What is the "velocity equivalent width"? What is the utility of computing and analyzing velocity equivalent widths? In words, qualitatively, describe what "∆v90" is measuring for a resolved absorption profile. Provide some insight on how ∆v90 is a more accurate representation of the true gas velocity spread for higher resolution spectra. 

15. In words, qualitatively, what does the D-index measure? What was the science motivation for the creation/development of the D-index? How is the D-index related to the saturation fraction, fsat , of a resolved absorption profile? 

16. Consider flux-decrement weighted velocity moments of resolved absorption profiles. What is a "kinematic subsystem"?  What does the 0th moment yield? What kinematics does the 1st moment quantify? What kinematics can be quantified using the 2nd moment? What kinematics can be quantified using the 3rd moment? 

17. In words, describe what a two-point (velocity) clustering functions (TPCF) measures?  What are some applications of TPCFs and what are they teaching us astrophysically? 

18. Describe what the composite Voigt profile TPCF (i.e., VP-TPCF) is measuring and what astrophysics it yields insight into. What advantages does the composite pixel-velocity TPCF (i.e., ∆vpix -TPCF) have over the composite VP-TPCF? 

Problems

1. Under construction