epoch of massive galaxy assembly

 Perhaps the key result from these early near-IR surveys was that, unexpectedly at the time, at a redshift of z=1 (at which point the Universe was basically half its current age) a large fraction of the most massive galaxies (galaxies containing the same stellar mass as 100 billion stars like our Sun) where already in existence. However, if you push your observations further back in time, out to perhaps a redshift of z=3 (when the Universe was only 15% of its current age), you find that only a small fraction of the massive galaxies we observe locally already existed at this earlier epoch. Consequently, it became clear that the 3.5 billion years of history contained within the redshift interval 1<z<3 constitutes the "epoch of massive galaxy assembly", during which the vast majority of the massive galaxies we observe in the local Universe were formed. The primary motivation for the Ultra Deep Survey was to study this key epoch in the evolution of the Universe.

From http://candels-collaboration.blogspot.com/2012/07/uds-ultra-deep-survey.html