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5.2. The Epoch of Galaxy Formation

Can the observed number count N(z) at high z (of a few) be compatible with Omegam = 1? This test can involve various objects such as quasars, early galaxies, damped Lyman alpha systems. etc.

New Developments: There are HST and Keck observations of central regions of galaxies in an early stage of their formation at z ~ 3 - 3.5 (some claim that galaxies are observed all the way to z ~ 6 based on ``photometric redshifts''). Improved spectrographs are being developed, as well as methods for estimating ``photometric redshifts''.

Pro: New data are accumulating rapidly from HST and 8-10 meter telescopes.

Con: The method is contaminated by unknown evolutionary issues. bullet The model predictions depend on the power spectrum of fluctuations. bullet Dissipationless simulations predict the number density of halos N(M, z), but not necessarily of luminous objects.

Current Results: The number count of quasars may be consistent with Omegam = 1 provided that there was efficient cooling and angular-momentum transfer [55]. bullet The abundance of bright galaxies seen at z ~ 3.5 [56] may favor low Omegam [57]. On the other hand, the relatively small number of galaxies identified in the Hubble Deep Field with colors consistent with very high redshifts (z = 4 - 6) [58] may indicate that low Omegam values can be excluded.