1.4. Confrontation of recent HST results with CDM models

The dominant dissipationless component of dark matter may accumulate by gravitational instability into clumps, the virialized parts of which would become dark matter halos of galaxies-to-be. Galaxies would then form by the cooling and condensation of gas within these halos. Hierarchical clustering currently appears to be the most successful at reproducing many of the properties of the real universe (RHS97), while remaining within the constraints produced by the COBE results. In the typical CDM model, structure grows from the gravitational collapse of small initial density perturbations, from which systems of progressively larger mass merge and collapse to form newly virialized systems. Observational evidence suggestive of hierarchical galaxy formation was presented by P96b, who found a significant number of sub-galactic sized star-forming objects to be gravitationally bound at z 2.39. P96b suggested that these "galaxy building blocks" are actually part of a widespread population existing throughout the redshift range z 2-5. P97 present a Cycle 6 HST project to image four random fields using the same F410M filter as P96b, to see if the faint Ly emitting candidates exist at z 2.4 in other fields in the sky (Section 3).