  High resolution, deep imaging surveys are instrumental in setting constraints
on semi-analytical structure formation models in Cold Dark Matter (CDM)
cosmologies. We show here that the lack of unresolved B-band ``dropouts'' with
V > 25 mag in the Hubble Deep Field (HDF) appears to be inconsistent with the
expected number of quasars if massive black holes form with a constant
universal efficiency in all CDM halos. To reconcile the models with the data, a
mechanism is needed that suppresses the formation of quasars in halos with
circular velocities v(circ) < 50-75 km/s. This feedback naturally arises due to
the photoionization heating of the gas by the UV background. We consider
several alternative effects that would help reduce the quasar number counts,
and find that these can not alone account for the observed lack of detections.
If reddening by dust can be neglected at early epochs, consistency with the
optical data also requires that the luminous extent of dwarf galaxies at high
redshifts be larger than a few percent of the virial radii of their dark matter
halos, in order not to overpredict the number of point-like B-band dropouts.
Future deep observations in the J and H bands with NICMOS might reveal several
z > 5 objects per field or provide even stronger constraints on the models than
existing B, V, and I data.
