Observable Properties of Cosmological Reionization Sources

Motivated by recent evidence that the epoch of reionization of hydrogen mayhave ended at a redshift as low as z~6 we consider the detectability of thesources responsible for this reionization. The main idea is that reionizationplaces limits on the mean surface brightness of the population of reionizationsources. Reducing the number of model-dependent assumptions to a minimum, wediscuss the observability of these sources with existing and plannedtelescopes. We define a family of models characterized by two parameters: theLyman continuum escape fraction f_c from the sources, and the clumpinessparameter C of the intergalactic medium. The minimum surface brightness modelcorresponds to a value of unity for both parameters. We find that the detectionof the non-ionizing UV continuum of the reionization sources will be difficultto accomplish before the launch of JWST if these sources have a mean surfacebrightness close to the minimum value. However, if the values of f_c and C aremore realistic, the reionization sources may well be detected by HST beforeJWST, perhaps by exploiting gravitational lensing amplification by foregroundclusters of galaxies. Instead of a detection in the continuum, one may attemptto detect the Lyalpha emission line by narrow-band imaging. Present Lyalphasearches at z>6 suggest that either the typical sources are fainter than dwarfgalaxies, or that the escape fraction of ionizing photons is much higher than50%, so that there are no bright compact HII regions formed around the ionizingsources.Comment: 28 pages, 6 figures. Accepted for publication in the Astrophysical Journal (to appear in vol. 600, N2, 10 January 2004