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).