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 ≤ 0.01Z⊙), 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–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 μm with the NGST, if they have a constant star formation rate of at least 100 M⊙/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.