Dark Matter Halo of Our Galaxy

Abstract

The fascination and the challenge for the search for the constituents of dark matter stem from the connection it bears with astronomy, astrophysics and cosmology on the one hand and with nuclear and particle physics on the other. Dark matter particles are the only relicts that still servive to-day from epochs prior to the primordial helium synthesis which took place at about 400s after the big bang. The first clues pointing to the presence of hidden mass in astronomical systems date-back to the 1920s and 1930s. Kepteyn (1922), Jeans (1922) and Oort (1930) after determining the mass density in the solar neighbourhood had noted that the visible stars contributed only a part to this measured value[1]. Zwicky (1933) measured the velocity dispersion of galaxies in the Coma-cluster (though based on observations of a small number of galaxies) and noted that a substantial part of the mass in the cluster has to be in some unseen form to account for the large observed value of the velocity dispersion[2]. This work was followed up by a few other workers and finally in 1972 based on observations of the velocities of hundreds of galaxies in the cluster, Rood et al., confirmed that atleast 75 % of the mass of the cluster was in some unseen form[3]. As the astronomical observations proceeded with increased vigor, very many ideas to explain the hidden mass were proposed. Among these the most exotic one (for that time) was presented by Cowsik & Mc Clelland (1972, 1973) [>4,5] who suggested that weakly interacting particles with finite rest-mass, generated in the early epochs of a hot big bang universe, would thermodynamically decouple from radiation and matter, and will evolve without substantial annihilation, to form a relict background of particles, which will gravitationally dominate over the normal baryonic matter, trigger the formation of galaxies, and thus generally form halos of invisible dark matter around galactic systems. This set of ideas has been developed and made more sophisticated by the work of many others and forms to-day the basic paradigm for the study of dark matter in the Universe.