Review Questions

1. What is the wavelength of the HeII Ly⍺ transition? The wavelengths of the HeII Balmer series are virtually identical to the wavelengths of the HI Lyman series. So, for intervening absorbing gas, why do we never need to worry that HI Lyman-series absorption lines are contaminated by HeII Balmer series absorption lines? 

2. Explain why, even if you have excellent UV spectrographs on space telescopes with sensitive coverage over the wavelength range 1100 Å to 1700 Å, it is very rare to find a quasar spectrum in which the HeII forest can be cleanly measured over the redshift range 3 ≤ z ≤ 4.

3. Briefly describe the impact on the shape and "hardness" of the UVB that HeI and HeII absorption can have. Include in your description the energy ranges that are most strongly modified by this absorption and by what factor they can be modified.  Why, for a fixed HI column density, do lower density clouds more strongly modify the shape of the UVB than higher density clouds?

4.  What is the hardness parameter (how is it defined)? Astrophysically, what is it measuring, meaning what is it telling you about the Universe?  What is the other ratio, which can be directly measured from a spectrum, that is used for this astrophysical measurement (HINT: it has been shown to be directly proportional to the hardness parameter)?

5. In the HeII Gunn-Peterson trough seen around z ≥ 3 strong transmission spikes can occur.  What is the interpretation of these HeII transmission spikes?  What evidence can we use from the associated HI absorption in the quasar spectrum to conclude that the ionizing spectrum in the physical region where the transmission spike is occurring is harder than the average background UVB?  What can we infer and/or deduce about the size and timescales of the source of this harder radiation?

6. Describe the Epoch of Helium Reionization. Include in your answer the approximated redshift range and the evolution of the IGM morphology and ionizing background spatial homogeneity. Why do we think the epoch begins when it does (in cosmic time and redshift)?  

7. Describe how the effective optical depth method, as applied to HeII transmission spikes, can be used to measure the redshift evolution of the HeII photoionization rate. Describe how this photoionization rate evolves toward the end of the Epoch of HeII Reionization.

Problems

1. Under construction