Reionization of the Universe
We know that at high redshift (z~1100) protons and electrons recombined
to form neutral hydrogen. This is the era of the microwave background.
But at z=0, most of the intergalactic medium (IGM) is ionized. What
happened? When did it happen?
We need a source of energetic photons: AGN or high mass stars
A little bit of energy goes a long way:
- nuclear fusion: 7x106 eV per H atom
- black hole accretion: 10x more
But it only takes 13.6 eV of energy to ionize H. So converting ~ 10-5
of the baryonic mass into stars or black holes will ionize the universe.
It takes place in a patchy process. Schematically (courtesy NASA/ESA):
So if we can measure the era of reioinization, we know when stars and
galaxies began to form in earnest.
1) Quasar Spectra and the Gunn-Peterson effect:
See reviews by Loeb
and Barkana 2001, Fan
et al 2006
If the IGM in the universe is neutral, any high energy photon will be
absorbed by hydrogen atoms in the ground state (n=1, Lyman series absorption). Even a small amount of neutral
hydrogen
can completely absorb Ly alpha photons from a source. Therefore if we
look back in redshift, the redshift where we see complete supression of
photons bluewards of Ly-alpha is the redshift at which reionization was
nearly complete.
QSO absorption line cartoon
low redshift vs high redshift quasar
spectra (de-redshifted; coudrtesy Bill Keel)
z=6.13 QSO (from Becker+ 2015),
showing the Gunn Peterson "trough":
Measure the amount of absorption shortward of Ly-alpha for many quasars
as a function of redshift....
...and turn it into a neutral hydrogen fraction of the universe:
So we see that reionization was nearly complete by z=6.
2. The Polarization of the Microwave Background
Photons scatter off of free electrons. Scattering induces
polarization of light. So microwave background photons traveling
through the re-ionized IGM will become polarized.
Multiple sources of scattering in the CMB: primordial signatures of
inflation (small angular scales), scattering by re-ionized IGM (larger
angular scales), scattering by Milky Way Galactic foreground
(correlated with Galactic sources). Search for polarization on large
scales after careful correction for foreground.
Modeling: Left: temperature
fluctuations, Right: Polarization power
(tau=scattering optical
depth; from Reichardt, crediting Lewis)
Observations (EE is polarization
power):
Current estimates of reionization redshift from Planck data (2016):
- reioinization redshift: z ~ 8-10 (t ~ 500 Myr)
- occurs in redshift span dz < 3 (dt ~ 250 Myr)
- universe less than 10% ioinized at z > 10
Earliest signatures of star formation (EDGES 21-cm absorption of CMB;
Bowman+18): z
~ 20-15 (t ~ 200 Myr)
Main
takeaway from reioinization: The universe stared forming stars early,
was forming
lots of
stars, galaxies, and quasars by redshifts z ~ 8-10, and these processes
had fully reionized the universe by z~6.
