The epoch of reionization marked the end of the ``dark ages'' during which
the ever-fading primordial background radiation cooled below 3000 K and shifted
first into the infrared and then into the radio. Darkness persisted until
early structures collapsed and cooled, forming the first stars and quasars
that lit the universe up again
[45].
The application of the Gunn-Peterson constraint on
the amount of smoothly distributed neutral material along the line of sight
to distant objects requires the hydrogen component of the diffuse IGM to
have been highly ionized by
z 
5
[47],
and the helium
component by z 2.5
[10].
From QSO absorption studies we also
know that neutral hydrogen at high-z accounts for only a small
fraction, ~ 10%, of the nucleosynthetic baryons
[30].
A substantial population of dwarf galaxies having star formation rates < 0.3 , and a space density in excess of that predicted by extrapolating to faint magnitudes the best-fit Schechter function, may be expected to form at early times in hierarchical clustering models, and has been recently proposed [39], [35], as a possible candidate for photoionizing the IGM at these early epochs. Establishing the character of cosmological ionizing sources is an efficient way to constrain competing models for structure formation in the universe, and to study the collapse and cooling of small mass objects at early epochs.
The study of the candidate sources of ionization at z = 5 can be
simplified by
noting that the breakthrough epoch (when all radiation sources can see
each other in the hydrogen Lyman-continuum) occurs much later in the
universe than
the overlap epoch (when individual ionized zones become simply
connected and every point in space is exposed to ionizing radiation).
This implies that at high redshifts the ionization equilibrium is actually
determined by the instantaneous UV production rate
[35]. The
fact that the IGM is rather clumpy and still optically thick at overlapping,
coupled to recent
observations of a rapid decline in the space density of radio-loud quasars
and of a large population of star-forming galaxies at z
3, has some
interesting implications for rival ionization scenarios and for the star
formation activity in the interval < 3 < z < 5.
The existence of a decline in the space density of bright quasars at redshifts beyond ~ 3 was first suggested by [40], and has been since then the subject of a long-standing debate. In recent years, several optical surveys have consistently provided new evidence for a turnover in the QSO counts [24], [55], [46], [28]. The interpretation of the drop-off observed in optically selected samples is equivocal, however, because of the possible bias introduced by dust obscuration arising from intervening systems. Radio emission, on the other hand, is unaffected by dust, and it has recently been shown [48] that the space density of radio-loud quasars also decreases strongly for z > 3. This argues that the turnover is indeed real and that dust along the line of sight has a minimal effect on optically-selected QSOs. In this case the QSO emission rate of hydrogen ionizing photons per unit comoving volume drops by a factor of 3 from z = 2.5 to z = 5, as shown in Figure 6.
|
Figure 6. Left: comoving space density of
bright QSOs as a
function of redshift. The data points with error bars are taken from
[24]
(filled dots),
[55]
(filled squares),
[46]
(crosses), and
[28]
(filled pentagon).
The empty triangles show the space density of radio-loud quasars
[26].
Right: comoving emission rate of hydrogen Lyman-continuum
photons (solid line) from QSOs, compared with the minimum rate
(dashed line) which is needed to fully ionize a fast recombining (with
gas clumping factor C = 30) EdS universe with
|
Galaxies with ongoing star-formation are another obvious source of
Lyman-continuum photons. Since the
rest-frame UV continuum at 1500 Å (redshifted into the visible band for a
source at z 3) is
dominated by the same short-lived, massive stars
which are responsible for the emission of photons shortward of the Lyman edge,
the needed conversion factor, about one ionizing photon every 10 photons at
1500 Å, is fairly insensitive to the assumed IMF and independent of the
galaxy history for t >> 107 yr.
Figure 6 (right) shows the
estimated Lyman-continuum luminosity density of galaxies at z
3.
The data point assumes a value of
fesc = 0.5 for the unknown fraction of ionizing
photons which escapes
the galaxy H I layers into the intergalactic medium. One
should note that, while highly reddened galaxies at high
redshifts would be missed by the Lyman-break color technique (which isolates
sources that have blue colors in the optical and a sharp drop in the
rest-frame UV), it seems unlikely that very dusty objects (with
fesc
<< 1) would contribute in any significant manner to the ionizing
metagalactic flux.
bh2 = 0.02. Models based on
photoionization by quasar sources
appear to fall short at z