COMMENT ON THE DISPERSION-VELOCITY OF GALACTIC DARK-MATTER PARTICLES

In a recent Letter, Cowsik, Ratnam and Bhattacharjee (1996a) have buil t a dynamically self-consistent spatial distribution of particles of g alactic dark matter. They have come up with the rather unorthodox conc lusion that the mean velocity dispersion of dark matter particles ''sh ould be 600 km s(-1) or larger''. Their letter triggered immediate com ments (Evans 1996, Gates et al. 1996). Here we find, as did Cowsik et al. (1996a), that models of the dark matter halo can be made consisten t with velocity dispersion much larger than that expected from a simpl e application of the virial theorem in the solar neighbourhood. But in contrast to their conclusions, we show that using their model, we als o obtain solutions with smaller velocity dispersion down to similar to 270 km s(-1). These more orthodox dispersions al ise because of less constraining boundary conditions for the central density but do not re ly on indirect or model dependent measurements of the large scale dist ance behaviour of the rotation curve.