2.4.1 The search for galaxies at very high redshifts The Lya emission line can be a strong feature in the far-UV spectrum of star-forming galaxies, when not obscured by dust, with equivalent width up to Ew - 150 A (see the discussion in Charlot & Fall 1993) or even larger in very young systems with top-heavy initial mass functions (IMFs) (e.g. Rhoads & Malhotra 2003). The Lya emission in AGNs is typically stronger than that in star-forming galaxies, with equivalent width up to a few times 103 A. In both cases, the line luminosities of both classes of sources, for redshifts up to z — 7, are within the sensitivity capabilities of modern telescopes and instrumentation. To illustrate this, let's consider the case of a star-forming galaxy with no dust obscuration, for which the expected luminosity density of the UV continuum at A — 1216 A is
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