3.6 Distinguishing Between Objects with z > zi and z < zi
The blanketing effect due to the Lyman alpha forest - known to be becoming denser towards higher redshifts, and likely therefore to be even thicker beyond z = 5 - would be severe, and would block out the blue wing of Lyman alpha emission from a high-z source. Such objects may still be best detected via their Lyman alpha emission even though the absorption cuts the equivalent width by half. But at redshifts larger than zi - in other words, before ionization breakthrough - the Gunn-Peterson optical depth is so large that any Lyman alpha emision line is blanketed completely, because the damping wing due to IGM absorption spills over into the red wing (Miralda-Escudé and Rees 1998). This means that any objects detectable beyond z would be characterised by a discontinuity at the redshifted Lyman alpha frequency. The Lyman alpha line itself would not be detectable (even though this may be the most prominent feature in objects with z < zi).