ObservationalCosmology

Self-test problems

Use these questions as you proceed through the unit, to judge whether your coverage of the material and level of understanding are adequate. Answers are just a click away, via the      button, but you will greatly reduce the diagnostic value of the questions if you look at the solutions before making a serious attempt to answer the question yourself.

1. What is the M/L ratio ? What units are usually used?  What would be the M/L ratio of a galaxy composed of 0.2 solar mass stars? 
2. Which single constraint on the amount of baryonic matters shows that (a) dark baryonic matter is required and (b) the dark baryonic matter density cannot equal the critical density? 
3. What key observation shows that there is dark matter in spiral galaxy halos?  In clusters of galaxies? 
4. If the mean blue luminosity density of the Universe is j_B =2.4x10⁸h  L_solMpc^-3 (where h=H₀/100 km/s/Mpc), calculate the mass-to-light ratio required to close the Universe if H₀=70 km/s/Mpc.
Why is the estimated value of j_B proportional to the assumed value of H₀? 
5. What are typical M/L values for the visible regions of galaxies ?  Using the answer from the previous question, what fraction of the closure density is in the visible regions of galaxies ? 
6. Why is the total M/L value for galaxies still subject to considerable uncertainty? 
7. How do estimates of the masses of galaxy clusters derived from (a) virial analyses, and (b) X-ray analyses assuming hydrostatic gas, scale with H₀? 
8. Stellar remnants such as neutron stars and black holes have extremely high M/L ratios. Why can these not provide the bulk of the dark matter? 
9. What is the essential difference between hot and cold dark matter? 
10. In what form are most of the observed baryons around us ? What is the total contribution to Omega of the observed baryons ? 


11. When a virial analysis is applied to a galaxy cluster to obtain M_v, what exactly has one derived the mass of?
    A similar approach can be applied to an elliptical galaxy. Would this give the mass of the whole galaxy? 
12. Combining Lyman alpha absorption studies with Big Bang nucleosynthesis results, what picture emerges about the likely state of the baryonic matter at z~3?