Quasars in the Early Universe: Smokestacks of the first Cosmic Cities?

A team of astronomers, including members of the Max Planck Institute for Astrophysics, have combined the observational power provided by the Hubble Space Telescope with the predictive power of the Millennium Run cosmological simulations to investigate an intriguing cosmic puzzle. If luminous quasars in the early Universe mark the regions that were the first to collapse and form massive clusters of galaxies as predicted by theory, then why is the observational evidence for such cosmic "cities-under-construction" currently so scarce?