I propose a unified framework for a joint analysis of the Drake equation and the Fermi paradox, which enables a simultaneous, quantitative study of both of them. The analysis is based on a simplified form of the Drake equation and on a fairly simple
scheme for the colonization of the Milky Way. It appears that for sufficiently long-lived civilizations, colonization of the Galaxy is the only reasonable option to gain knowledge about other life forms. This argument allows one to define a region in the parameter space of the Drake equation where the Fermi paradox definitely holds (Strong Fermi paradox).
To account for the observed differential metallicity distribution (DMD) of the Milky Way halo, a semi-analytical model is presented in the framework of the hierarchical merging paradigm for structure formation. It is assumed that the Milky Way halo i
s composed of a number of sub-haloes with properties either as observed in the dwarf satellite galaxies of the Local group (shape of metallicity distribution, effective yield) or derived from calculations of structure formation (sub-halo distribution function). With reasonable assumptions for the parameters involved, we find that the overall shape and effective yield of the Galactic halo DMD can be reproduced in the framework of such a simple model. The low metallicity tail of the DMD presents a defficiency of stars with respect to the simple model predictions (akin to the G-dwarf problem in the solar neighborhood); it is suggested that an early infall phase can account for that problem, as well as for the observed DMDs of dwarf satellite galaxies.Accretion of galaxies similar (but not identical) to the progenitors of present day dwarf satellites of the Milky Way may well have formed the Galactic halo.
