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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 approx 5 [47], and the helium component by z approx 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 gtapprox 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

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 Omegabh2 = 0.02. Models based on photoionization by quasar sources appear to fall short at z approx 5. The data point shows the estimated contribution from star-forming galaxies at z approx 3, assuming that the fraction of Lyman continuum photons which escapes the galaxy H I layers into the intergalactic medium is 50% (see [35] for details).

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 approx 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 approx 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.

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