7. Reionization Flashcards
What was the redshift at the bigbang and recombination?
Infinity at the BB
1100 at recombination
What is recombination?
p+e -> H I
What are Stromgren spheres?
An ionised bubble for a source emitting N dot photons per second
What is the clumping factor
Measure of the density of hydrogen gas
- Compares the density of protons and electrons vs electrons ^2
What is the filling factor?
What fraction of the volume of the universe is ionised
What does it mean if we have quantities represented as <x>?</x>
They are volume averaged quantities
As the Universe expands, what happens to the number density <n>, clumping factor and recombination timescale? What is a consequence of this with regards to electrons and ionisation?</n>
<n> and clumping factor decrease
t_rec increases
- Electrons are far less likely to recombine as time passes (low z)
- Universe stays ionised even after bright hot sources fade
</n>
What is the Gunn-Peterson trough?
Occurs when Lyman alpha forest absorption lines merge into continuous absorption
- Light will be scattered or reabsorbed by electrons
What is the optical depth (tau) and how can we measure it?
It is a measure of how much light is absorbed when light travels through an absorbing medium
- Measure it by comparing observations to models
When inserting reasonable parameters in the optical depth equation, what conclusion can be drawn?
We get that the optical depth, tau, is > > 1 implying total absorption
- Ly alpha absorption is sensitive to v small neutral fractions
What does the presence of total absorption (GP troughs) for distant objects (give examples) tell us about the Universe?
Examples: QSOs (z > 6) and GRBs
- Shows that the U has just become ionised with a small neutral fraction left
What is the main assumption which is slightly incorrect that we used in the optical depth equation?
Assumed gas is static but it is not
- Ly alpha absorption is never a perfect delta function
To get around this, we can model the effects of dense gas to be slightly wavelength dependent yielding “damping wings”
What are damping wings?
When we suppress flux either side of the absorption wavelength
How are damping wings obseved for bright distant sources?
As a curved edge to the Ly alpha break
- Can also suppress Ly alpha emission from the original source
How does the GP trough affect the type of galaxies that we observe?
Distant galaxies are expected to be Ly alpha emitters
- Fewer will be than expected but we will still see Ly break galaxies
