Epoch of galaxy formation

There are two distinct phases related to the formation epoch of galaxies. In an expanding universe, a galaxy's birth would mark the time when a system turned around under its own self gravity and acquired a separate dynamical status. A second stage more amenable to searches for young galaxies marks the epoch when such a system undergoes its first primary episode of star formation. On the basis of cooling arguments and dynamical timescales, it is usual to assume that the former phase precedes the latter i.e. majority of stars form within a self-gravitating gaseous 'protogalaxy'. In this paper I illustrate how the pre-existence of a central black hole and its activity influences the protogalactic environment to initiate large scale star formation.