and Cosmology

9.4 Reionization of the Universe
9.4.2 The Reionization Process
Dissociation of Molecular Hydrogen. The energetic
photons from these Population III stars are now capable
of ionizing hydrogen in their vicinity. More important
still is another effect: the binding energy of H 2 is only
11.26 eV. Since the Universe is transparent for photons
with energies below 13.6 eV, photons with 11.26 eV ≤
E γ ≤ 13.6 eV can propagate very long distances and
dissociate molecular hydrogen. This means that as soon
as the first stars have formed in a region of the Universe,
molecular hydrogen in their vicinities will be destroyed
and further star formation will then be prevented. 2
Metal Enrichment of the Intergalactic Medium.
Soon after Population III stars have formed, they will
explode as supernovae. Through this process, the metals
produced by them are ejected into the intergalactic
medium, by which the initial metal enrichment of the
IGM occurs. The kinetic energy transferred by SNe to
the gas within the halo can exceed its binding energy,
so that the baryons of the halo can be blown away and
further star formation is prevented. Whether this effect
may indeed lead to gas-free halos, or whether the released
energy can instead be radiated away, depends
on the geometry of the star-formation regions. In any
case, it can be assumed that in those halos where the
first generation of stars was born, further star formation
was considerably suppressed, particularly since all
molecular hydrogen was destroyed.
We can assume that the metals produced in these first
SN explosions are, at least partially, ejected from the
halos into the intergalactic medium, thus enriching the
latter. The existence of metal formation in the very early
Universe is concluded from the fact that even sources at
very high redshift (like QSOs at z ∼ 6) have a metallicity
of about one tenth the Solar value. Furthermore, the Lyα
forest also contains gas with non-vanishing metallicity.
Since the Lyα forest is produced by the intergalactic
medium, this therefore must have been enriched.
The Final Step to Reionization. For gas to cool in halos
without molecular hydrogen, their virial temperature
needs to exceed about 10 4 K (see Fig. 9.29). Halos of
2 To destroy all the H 2 in the Universe one needs less than 1% of the
photon flux that is required for the reionization.
this mass form with appreciable abundance at redshifts
of z ∼ 10, as follows, e.g., from the Press–Schechter
model (see Sect. 7.5.2). In these halos, efficient star formation
can then take place; the first proto-galaxies will
form. These will then ionize the surrounding IGM in
the form of HII regions, as sketched in Fig. 9.30. The
corresponding HII regions will expand because increasingly
more photons are produced. If the halo density is
sufficiently high, these HII regions will start to overlap.
Once this occurs, the IGM is ionized, and reionization
is completed.
We therefore conclude that reionization is a twostage
process. In a first phase, Population III stars form
through cooling of gas by molecular hydrogen, which
is then destroyed by these very stars. Only in a later
epoch and in more massive halos is cooling provided by
atomic hydrogen, then leading to reionization.
A Luminous J-band Drop-Out? The aforementioned
fact that, even at redshifts as large as z ∼ 6, massive
galaxies with a fairly old stellar population were already
in place shows that there was an epoch of intense star
formation at even earlier times. This clearly suggests
that these galaxies must have played an important role
in the reionization process of the Universe. In fact, in
the HUDF an object was found that appears to be a J-
band drop-out, with no radiation seen at wavelengths
shorter than the J-band. Spectroscopy of this source
revealed the presence of a strong 4000-Å break, not only
giving further support to its high redshift of z ∼ 6.5, but
also indicating that the source contains a post-starburst
stellar population. The bolometric luminosity of this
source is estimated to be ∼ 10 12 L ⊙ , and using a Salpeter
initial mass function (see Eq. 3.67), a stellar mass of
∼ 6 × 10 11 M ⊙ is obtained. The strong 4000-Angstrom
break indicates that the spectral energy distribution is
dominated by A0 stars of masses 3M ⊙ . This provides
a clear indication of an old age of the population of
300 Myr, implying that the stars must have formed at
redshifts z > 9, but possibly at even higher redshift.
We can estimate the comoving volume that this galaxy
was able to reionize, based on its high luminosity.
With all uncertainties entering such an estimate (such as
the escape fraction of ionizing photons from the galaxy),
it is concluded that this galaxy could ionize a volume
of ∼ 10 5 Mpc 3 . This needs to be compared to the highredshift
volume within which such a source would have
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