The distribution of dark and luminous matter inferred from extended rotation curves

Abstract

A better understanding of the formation of mass structures in the Universe can be obtained by determining the amount and distribution of dark and luminous matter in spiral galaxies. To investigate such matters a sample of 12 galaxies, most with accurate distances, has been composed of which the luminosities are distributed regularly over a range spanning two and a half orders of magnitude. Of the observed high quality and extended rotation curves of these galaxies decompositions have been made, for four different schemes, each with two free parameters. For a ‘maximum disc fit’ the rotation curves can be well matched, yet a large range of mass-to-light (M/L) ratios for the individual galaxies is required. For the alternative gravitational theory of MOND (Modified Newtonian Dynamics) the rotation curves can be explained if the fundamental parameter associated with MOND is allowed as a free parameter. Fixing that parameter leads to a disagreement between the predicted and observed rotation curves for a few galaxies. When cosmologically motivated NFW dark matter haloes are assumed, the rotation curves for the least massive galaxies can, by no means, be reproduced; cores are definitively preferred over cusps. Finally, decompositions have been made for a pseudo-isothermal halo combined with a universal M/L ratio. For the latter, the light of each galactic disc and bulge has been corrected for extinction and has been scaled by the effect of stellar population. This scheme can successfully explain the observed rotations and leads to submaximum disc mass contributions. Properties of the resulting dark matter haloes are described and a ratio between dark and baryonic mass of ∼9 for the least, and of ∼5, for the most luminous galaxies has been determined, at the outermost measured rotation.