Primordial Proto-Galaxies

The first stars to form in protogalaxies must have primordial chemical composition. We refer to a protogalaxy that is forming stars of primordial composition, or very low metallicity ($Z\le 0.01 Z_{\odot}$), as ``Primordial Proto-Galaxy'' (PPG). PPGs contain little or no dust, and therefore their spectral energy distribution can be modelled from the rest-frame ultraviolet to the infrared without accounting for dust extinction and emission. We present the results of computing the photometric properties of high redshift PPGs at near-d-to-mid infrared wavelengths, that will soon be available with the new generation of infrared space telescopes, such as SIRTF and the NGST. We show that: i) PPGs at very high redshift (5<z<10) should be easily selected from deep near/mid IR surveys with a colour-colour criterion; ii) PPGs at redshift 5<z<10 can be detected at 8 $\mu$m with the NGST, if they have a constant star formation rate of at least 100 M$_{\odot}/$yr; iii) once the redshift of a PPG photometric candidate is determined, its near-to-mid infrared colors should provide strong constraints on the stellar IMF at zero or very low metallicity.