Galaxy Formation: ISOCAM Counts

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Abstract
Detailed observations in the optical do not allow to discriminate between the following scenarii of galaxy formation: - synchronous formation at large redshift (z > 7). - hierarchical merging, down to z ≤ 1, of proto-galactic lumps formed at very high redshift (≈ 10). - galaxy formation at low z (≈ 2-5), but partially obscured by dust. More than 1500 galaxies in a field of 2.6′ × 4.6′ were detected by Tyson (1988) with CCD images in the B band. These deep counts of galaxies revealed the presence of an unexpectedly large population of blue galaxies, corresponding to a number excess of 3 to 5 times more galaxies at B > 23 than one can expect from the local luminosity function. These objects lie apparently at moderate redshift (z = 0.3 for B = 23–24) as shown by spectroscopy of B-selected galaxies (Colless et al. 1990).

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