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:
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 first began to form.

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 (>13.6 eV, Ly alpha) will be absorbed. 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.

z=3.62 QSO spectrum, showing the Ly-alpha forest from intervening neutral H clouds:

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.

(from Reichardt, crediting Lewis)

Modeling:  Left: temperature fluctuations, Right: Polarization power

Observations (EE is polarization power):

Current estimates of reionization redshift from Planck data (2016):

Main takeaway from reioinization: The universe as a whole began forming stars and galaxies at redshifts z ~ 8-10, and these processes had fully reionized the universe by z~6.